Lithium ion battery using crosslinkable separator

ABSTRACT

A separator for an electricity storage device comprising a silane-modified polyolefin, wherein silane crosslinking reaction of the silane-modified polyolefin is initiated when it contacts with the electrolyte solution, as well as a method for producing the separator.

FIELD

The present invention relates to a separator for an electricity storagedevice and to a crosslinking method for it, and to an electricitystorage device assembly kit and a method for producing the electricitystorage device.

BACKGROUND

Microporous membranes are widely used as membranes for separation orselective permeation and selection of various substances and asisolating materials, and some examples of their uses include asmicrofiltration membranes, as fuel cell and condenser separators, or asmatrices for functional membranes or separators for electricity storagedevices, that exhibit new functions by having functional materialspacked into their pores. Polyolefin microporous membranes, specifically,are preferred for use as separators for lithium ion batteries that arewidely utilized in PC laptops, cellular phones and digital cameras.

In order to ensure battery safety, separators must have both an activeshutdown function and high membrane rupture temperature. PTL 1, forexample, describes adjustment of the higher physical properties of apolyolefin resin as an essential component of a separator for a lithiumion battery. In addition, as described in PTL 2, it is known that heatrelease due to interior battery short circuiting is inhibited by ashutdown function when the degree of crystallinity and gel fraction arein specific ranges, and that the safety of a battery can be ensured ifit has performance such that a membrane rupture does not occur in thebattery cell at partial high temperature sections (i.e. breakdown at170° C. or higher). More specifically, in regard to PTLs 1 and 2, it hasgradually come to light by experimentation that high-temperaturemembrane rupture properties can be exhibited by constructing silanecrosslinked sections (a gel structure) in a polyolefin separator.

PTLs 1 to 6, for example, describe a silane crosslinking structureformed by contact between a silane-modified polyolefin-containingseparator and water. PTL 8 describes a crosslinked structure formed fromring-opening of norbornane by irradiation with ultraviolet rays or anelectron beam. PTL 9 describes a separator insulating layer having a(meth)acrylic acid copolymer with a crosslinked structure, and astyrene-butadiene rubber binder. A separator has also been proposed thathas a layer A having a shutdown property and a layer B comprising anaramid resin and an inorganic material, with the ratio of theirthicknesses adjusted to within a prescribed range (see PTL 11).

The members used in a lithium ion battery are a positive electrode, anegative electrode material, an electrolyte solution and a separator.Among these members, the separator must be inactive from the standpointof electrochemical reaction and with respect to the peripheral members,because of its role as an insulating material. Since development of thefirst negative electrode materials for lithium ion batteries, a methodhas been established for inhibiting decomposition of the electrolytesolution on the negative electrode surface, wherein a solid electrolyteinterface (SEI) is formed by chemical reaction during initial charge(NPL 1). Even when a polyolefin resin is used as the separator, somecases have been reported in which oxidation reaction is induced on thepositive electrode surface at high voltage, resulting in blackening orsurface degradation of the separator.

For this reason, the materials used for electricity storage deviceseparators are designed with chemical structures that are inert inelectrochemical reactions or other chemical reactions, and as a result,polyolefin microporous membranes have become widely developed andimplemented. When a polyolefin is used as the resin, however, theimprovement in performance has been limited even when the mechanicalmicropore structure of the separator is modified. For example, when theperformance is insufficient in terms of the heat-resistant stability ofthe separator at above the melting point of the polyolefin, or in termsof the affinity with the electrolyte solution or the liquid retentiondue to electronegativity of the olefin units, it is not possible toobtain satisfactory permeability of Li ions or their solvated ionclusters in the separator.

Because of such limitations, therefore, by current means it is notpossible to satisfy the high-speed charge-discharge or heat stabilityrequired for modern battery development.

CITATION LIST Patent Literature

-   [PTL 1] Japanese Unexamined Patent Publication No. H09(1997)-216964-   [PTL 2] International Patent Publication No. WO97/44839-   [PTL 3] Japanese Unexamined Patent Publication No. H11(1999)-144700-   [PTL 4] Japanese Unexamined Patent Publication No. H11(1999)-172036-   [PTL 5] Japanese Unexamined Patent Publication No. 2001-176484-   [PTL 6] Japanese Unexamined Patent Publication No. 2000-319441-   [PTL 7] Japanese Unexamined Patent Publication No. 2017-203145-   [PTL 8] Japanese Unexamined Patent Publication No. 2011-071128-   [PTL 9] Japanese Unexamined Patent Publication No. 2014-056843-   [PTL 10] Japanese Unexamined Patent Publication No. H10(1998)-261435-   [PTL 11] Japanese Unexamined Patent Publication No. 2007-299612-   [PTL 12] International Patent Publication No. WO2010/134585-   [PTL 13] Japanese Unexamined Patent Publication No. 2016-072150

Non Patent Literature

-   [NPL 1] Lithium Ion Secondary Batteries (2nd Edition) Nikkan Kogyo    Shimbun, Ltd.-   [NPL 2] Kiso Kobunshi Kagaku, Tokyo Kagaku Dojin

SUMMARY Technical Problem

With the increasing high outputs and high energy densities of lithiumion secondary batteries for mobile devices and vehicles in recent years,there is ongoing demand for smaller battery cell sizes and for stablecycle charge-discharge performance during long periods of use. It istherefore considered necessary for the separators used to bethin-membranes (for example, 15 μm or smaller) with high quality (forexample, homogeneous physical properties and free of resin aggregates).Standards have also become more rigorous for battery safety, and as alsodescribed in PTLs 1 and 2, there is a need for shutdown functions andhigh-temperature membrane rupture properties, while expectations arealso high for development of separator resin compositions that can bestably produced, and production methods for them. In this regard, thelevel for shutdown temperature is preferably as far below 150° C. aspossible, while the membrane rupture temperature is preferably as high atemperature as possible.

In the method described in PTL 3, for example, a crosslinking catalystmaster batch is used during the extrusion step to promotesilane-modified polyethylene crosslinking reaction in the extruder, butthis results in generation of resin aggregates and lowers thehomogeneity of the physical properties of the separator. As a solutionfor this problem, the methods described in PTLs 4, 5 and 6 proposeproviding a plasticizer extraction step or silane gel crosslinking step,or controlling the gel fraction of the resin membrane, or dewateringafter casting of the uncrosslinked resin through hot water. In addition,PTL 7 proposes a polyolefin microporous membrane with modification ofthe gel fraction, the storage modulus at 40° C. to 250° C. based ondynamic viscoelasticity (DMA) measurement, the maximum shrinkage factorbased on thermomechanical analysis (TMA) and amount of radicals asmeasured by electron spin resonance (ESR), to provide a heat-resistantresin microporous membrane with low heat shrinkage, low fluidity andexcellent meltdown resistance.

In addition, from the viewpoint of dimensional stability, and of bothmaintaining the shutdown function and increasing the membrane rupturetemperature for separators for electricity storage devices, it has beenproposed to provide an inorganic porous layer containing inorganicparticles such as calcined kaolin or boehmite and a resin binder on atleast one surface of a polyolefin microporous membrane (PTLs 12 and 13).

However, the method disclosed in PTL 4 is not able to sufficientlypromote silane crosslinking reaction, and it is difficult to obtainhigh-temperature membrane rupture resistance. Crosslinking reaction canbe promoted in the plasticizer extraction steps described in PTLs 3 and4 since they employ a tin(II)-based crosslinking catalyst, but there areconcerns regarding post-residue of the crosslinking catalyst.

The heat-resistant resin microporous membrane described in PTL 7 ismerely obtained by coating a photopolymerizable coating solution onto adry porous membrane. In Example 5 of PTL 7 a low-molecular-weight silanecoupling agent such as γ-methacryloxypropyltrimethoxysilane is added tothe porous membrane, but when a low-molecular-weight silane couplingagent is used in a wet porous method, it is expected that thelow-molecular-weight silane coupling agent does not bond with the resinof the porous membrane since it tends to react or bond with thepore-forming plasticizer. A battery comprising a heat-resistant resinmicroporous membrane such as described in PTL 7 as the separator haspoor cycle characteristics, and when used for prolonged periods,unpredictable secondary reactions may be induced in the battery,potentially lowering the battery safety.

Moreover, the skin layer described in PTL 7 is formed by coating acompound with a polymerizable functional group onto a resin porousmembrane followed by crosslinking reaction by external stimulation, andtherefore some infiltration into the resin porous membrane is expectedto occur during coating of the skin layer, and a mixed region isexpected to form near the interface between the skin layer and in theresin porous membrane after the crosslinking reaction has proceeded.This allows satisfactory TMA heat shrinkage performance to be obtained,but is also expected to lead to lower battery cycle characteristics dueto blockage of the resin porous membrane, or reduced fuse (shutdown)performance as the resin porous membrane undergoes melting. In addition,small amounts of radical species are detected by ESR and remain in thecomposite microporous membrane obtained by the method described in PTL7, and when such a composite microporous membrane has been incorporatedinto a battery, radical reaction would be expected to take place withthe other members and particularly the electrolyte solution, resultingin chain reaction that would decompose the electrolyte solution and thuspotentially resulting in notable impairment of the battery performance.

Furthermore, the microporous membranes and separators described in PTLs1, 2 and 7 have been poorly studied in terms of placing inorganic porouslayers comprising inorganic particles and a resin binder on theirsurfaces. A conventional separator comprising an inorganic porous layeron a microporous membrane will appear to have an improved membranerupture temperature in the temperature-resistance curve of anelectricity storage device. In practice, however, since the resin oftenelutes from the microporous membrane into the inorganic porous layer,loss of the membrane and a resulting reduction in stress resistance areto be expected for the separator as a whole. The multilayer porousmembranes described in PTLs 12 and 13 are therefore provided with apolyolefin microporous membrane and an inorganic porous layer, but thereis still room for investigation regarding both the low temperatureshutdown function and high-temperature membrane rupture properties as aseparator for an electricity storage device, and regarding improvedelectricity storage device cycle characteristics and battery nailpenetration safety.

A battery using a separator such as described in PTLs 3 to 7 has poorcycle characteristics, and when used for prolonged periods,unpredictable secondary reactions may be induced in the battery,potentially lowering the battery safety.

With common molded articles such as hot water pipes, a tin (Sn)-basedcatalyst is loaded into the extruder during the extrusion step. A wetproduction process for a separator for an electricity storage device, onthe other hand, usually includes steps such as extrusion and sheetforming, stretching, plasticizer extraction (pore formation), heattreatment, and winding, and therefore when silane crosslinking isaccelerated in the extruder during the sheet-forming step, it can leadto production defects in the gelled portions and difficult stretching ofthe silane-crosslinked polyolefin in the subsequent stretching step.More study is therefore necessary to obtain a novel separator for anelectricity storage device suited for the production process.

Moreover, the crosslinking methods described in PTLs 1 to 6, 8 and 9 areall in-processes for separator membrane formation, or are carried out ina batch process immediately after separator membrane formation. Afterformation of a crosslinked structure as described in PTLs 1 to 6, 8 and9, therefore, it is necessary to coat or to form slits in the separator,which increases the internal stress during the subsequent layering andwinding steps with the electrodes and can lead to deformation of thefabricated battery. For example, when a crosslinked structure is formedby heating, internal stress in the separator with the crosslinkedstructure often increases at ordinary temperature or room temperature.

When a crosslinked structure is formed by photoirradiation ofultraviolet rays or an electron beam, the light irradiation may benon-uniform and the crosslinked structure may become nonhomogeneous.This is believed to occur because the peripheries of the crystals of theresin forming the separator tend to become crosslinked by an electronbeam.

Incidentally, PTL 10 describes a technique for improving the cyclecharacteristics of a lithium ion secondary battery by addition of asuccinimide or the like to the electrolyte solution. However, thetechnique described in PTL 10 is not one that improves the cyclecharacteristics by specifying the structure of the separator.

With the separators for electricity storage devices described in PTLs 1,2 and 11, there is still room for improvement from the viewpoint ofimproving their electricity storage device performance.

In light of the problems described above, it is an object of the presentinvention to provide a separator for an electricity storage device thatcan exhibit both a shutdown function and high-temperature membranerupture resistance, while ensuring electricity storage device safety,output and/or cycle stability, as well as a novel crosslinking methodsuitable for its production, and an assembly kit or production methodfor the electricity storage device.

Solution to Problem

The aforementioned problems are solved by the following technical means.

[1]

A separator for an electricity storage device comprising asilane-modified polyolefin, wherein silane crosslinking reaction of thesilane-modified polyolefin is initiated when the separator for anelectricity storage device contacts with an electrolyte solution.

[2]

The separator for an electricity storage device according to [1] above,wherein the silane-modified polyolefin is not a master batch resincontaining a dehydrating condensation catalyst that crosslinks thesilane-modified polyolefin.

[3]

The separator for an electricity storage device according to [1] or [2]above, wherein the separator for an electricity storage device comprisespolyethylene in addition to the silane-modified polyolefin.

[4]

The method for producing a separator for an electricity storage deviceaccording to [3] above, wherein the weight ratio of the silane-modifiedpolyolefin and the polyethylene (silane-modified polyolefinweight/polyethylene weight) is 0.05/0.95 to 0.40/0.60.

[5]

A separator for an electricity storage device, comprising 5 to 40 weight% of a silane-modified polyolefin and 60 to 95 weight % of a polyolefinother than the silane-modified polyolefin, wherein the storage moduluschange ratio (R_(ΔE′)) is 1.5 to 20, as defined by the following formula(1):R _(ΔE′) =E′ _(S) /E′ _(j)  (1)where E′_(j) is the storage modulus measured at 160° C. to 220° C. forthe separator for an electricity storage device before crosslinkingreaction of the silane-modified polyolefin, E′_(S) is the storagemodulus measured at 160° C. to 220° C. for the separator for anelectricity storage device after crosslinking reaction of thesilane-modified polyolefin, and the measuring conditions for the storagemodulus E′ (E′_(j) or E′_(S)) are specified by the following (i) to(iv):

(i) the dynamic viscoelasticity measurement is carried out under thefollowing conditions:

-   -   Measuring apparatus: RSA-G2 (TA Instruments)    -   Sample thickness: from 5 μm to 50 μm    -   Measuring temperature range: −50 to 225° C.    -   Temperature-elevating rate: 10° C./min    -   Measuring frequency: 1 Hz    -   Transform mode: sine wave tension mode (linear tension)    -   Initial static tensile load: 0.5 N    -   Initial gap distance (at 25° C.): 25 mm    -   Auto strain adjustment: Enabled (range: 0.05 to 25% amplitude,        0.02 to 5 N sine wave load);

(ii) the static tensile load is the median value of the maximum stressand minimum stress for each periodic motion, and the sine wave load isthe vibrational stress centered on the static tensile load.

(iii) the sine wave tension mode is measurement of the vibrationalstress while carrying out periodic motion at a fixed amplitude of 0.2%,wherein in sine wave tension mode, the vibrational stress is measuredwhile varying the gap distance and static tensile load so that thedifference between the static tensile load and the sine wave load iswithin 20%, and when the sine wave load is 0.02 N or lower, thevibrational stress is measured while amplifying the amplitude value sothat the sine wave load is no greater than 5 N and the increase in theamplitude value is no greater than 25%, and

(iv) the storage modulus E′ is calculated from the relationship betweenthe obtained sine wave load and amplitude value, and the followingformulas:σ*=σ₀·Exp[i(ωt+δ)],ε*=ε₀·Exp(iωt),σ*=E*·ε*E*=E′+iE″where σ*: vibrational stress, ε*: strain, i: imaginary number unit, ω:angular frequency, t: time, δ: phase difference between vibrationalstress and strain, E*: complex modulus, E′: storage modulus, E″: lossmodulus,

vibrational stress: sine wave load/initial cross-sectional area

static tensile load: load at minimum point of vibrational stress foreach period (minimum point of gap distance for each period), and

sine wave load: difference between measured vibrational stress andstatic tensile load.

[6]

A separator for an electricity storage device, comprising 5 to 40 weight% of a silane-modified polyolefin and 60 to 95 weight % of a polyolefinother than the silane-modified polyolefin, wherein the loss moduluschange ratio (R_(ΔE″)) is 1.5 to 20, as defined by the following formula(3):R _(ΔE″) =E″ _(S) /E″ _(j)  (3)where E″_(j) is the loss modulus measured at 160° C. to 220° C. for theseparator for an electricity storage device before crosslinking reactionof the silane-modified polyolefin, E″_(S) is the loss modulus measuredat 160° C. to 220° C. for the separator for an electricity storagedevice after crosslinking reaction of the silane-modified polyolefin,and the measuring conditions for the loss modulus E″ (E″_(j) or E″_(S))are specified by the following (i) to (iv):

(i) the dynamic viscoelasticity measurement is carried out under thefollowing conditions:

-   -   Measuring apparatus: RSA-G2 (TA Instruments)    -   Sample thickness: from 5 μm to 50 μm    -   Measuring temperature range: −50 to 225° C.    -   Temperature-elevating rate: 10° C./min    -   Measuring frequency: 1 Hz    -   Transform mode: sine wave tension mode (linear tension)    -   Initial static tensile load: 0.5 N    -   Initial gap distance (at 25° C.): 25 mm    -   Auto strain adjustment: Enabled (range: 0.05 to 25% amplitude,        0.02 to 5 N sine wave load);

(ii) the static tensile load is the median value of the maximum stressand minimum stress for each periodic motion, and the sine wave load isthe vibrational stress centered on the static tensile load;

(iii) the sine wave tension mode is measurement of the vibrationalstress while carrying out periodic motion at a fixed amplitude of 0.2%,wherein in sine wave tension mode, the vibrational stress is measuredwhile varying the gap distance and static tensile load so that thedifference between the static tensile load and the sine wave load iswithin 20%, and when the sine wave load is 0.02 N or lower, thevibrational stress is measured while amplifying the amplitude value sothat the sine wave load is no greater than 5 N and the increase in theamplitude value is no greater than 25%; and

(iv) the loss modulus E″ is calculated from the obtained sine wave loadand amplitude value, and the following formulas:σ*=σ₀·Exp[i(ωt+δ)],ε*=ε₀·Exp(iωt),σ*=E*·ε*E*=E′+iE″where σ*: vibrational stress, ε*: strain, i: imaginary number unit, ω:angular frequency, t: time, δ: phase difference between vibrationalstress and strain, E*: complex modulus, E″: storage modulus, E″: lossmodulus,

vibrational stress: sine wave load/initial cross-sectional area

static tensile load: load at minimum point of vibrational stress foreach period (minimum point of gap distance for each period), and

sine wave load: difference between measured vibrational stress andstatic tensile load.

[7]

A separator for an electricity storage device comprising asilane-modified polyolefin, wherein silane crosslinking reaction of thesilane-modified polyolefin takes place when the separator for anelectricity storage device contacts with an electrolyte solution.

[8]

A separator for an electricity storage device, comprising 5 to 40 weight% of a silane-modified polyolefin and 60 to 95 weight % of a polyolefinother than the silane-modified polyolefin, wherein the mixed storagemodulus ratio (R_(E′mix)) is 1.5 to 20, as defined by the followingformula (2):R _(E′mix) =E′ _(a) /E′ ₀  (2)where E′_(a) is the storage modulus measured at 160° C. to 220° C. forthe separator for an electricity storage device, E′₀ is the storagemodulus measured at 160° C. to 220° C. for a separator for anelectricity storage device not containing the silane-modifiedpolyolefin, and the measuring conditions for the storage modulus E′(E′_(a) or E′₀) are specified by the following (i) to (iv):

(i) the dynamic viscoelasticity measurement is carried out under thefollowing conditions:

-   -   Measuring apparatus: RSA-G2 (TA Instruments)    -   Sample thickness: from 5 μm to 50 μm    -   Measuring temperature range: −50 to 225° C.    -   Temperature-elevating rate: 10° C./min    -   Measuring frequency: 1 Hz    -   Transform mode: sine wave tension mode (linear tension)    -   Initial static tensile load: 0.5 N    -   Initial gap distance (at 25° C.): 25 mm    -   Auto strain adjustment: Enabled (range: 0.05 to 25% amplitude,        0.02 to 5 N sine wave load);

(ii) the static tensile load is the median value of the maximum stressand minimum stress for each periodic motion, and the sine wave load isthe vibrational stress centered on the static tensile load;

(iii) the sine wave tension mode is measurement of the vibrationalstress while carrying out periodic motion at a fixed amplitude of 0.2%,wherein in sine wave tension mode, the vibrational stress is measuredwhile varying the gap distance and static tensile load so that thedifference between the static tensile load and the sine wave load iswithin 20%, and when the sine wave load is 0.02 N or lower, thevibrational stress is measured while amplifying the amplitude value sothat the sine wave load is no greater than 5 N and the increase in theamplitude value is no greater than 25%; and

(iv) the storage modulus E′ is calculated from the relationship betweenthe obtained sine wave load and amplitude value, and the followingformulas:σ*=σ₀·Exp[i(ωt+δ)],ε*=ε₀·Exp(iωt),σ*=E*·ε*E*=E′+iE″where σ*: vibrational stress, ε*: strain, i: imaginary number unit, ω:angular frequency, t: time, δ: phase difference between vibrationalstress and strain, E*: complex modulus, E′: storage modulus, E″: lossmodulus,

vibrational stress: sine wave load/initial cross-sectional area

static tensile load: load at minimum point of vibrational stress foreach period (minimum point of gap distance for each period), and

sine wave load: difference between measured vibrational stress andstatic tensile load.

[9]

A separator for an electricity storage device, comprising 5 to 40 weight% of a silane-modified polyolefin and 60 to 95 weight % of a polyolefinother than the silane-modified polyolefin, wherein the mixed lossmodulus ratio (R_(E″mix)) is 1.5 to 20.0, as defined by the followingformula (4):R _(E″mix) =E″ _(a) /E″ ₀  (4)where E″_(a) is the loss modulus measured at 160° C. to 220° C. for theseparator for an electricity storage device, E″₀ is the loss modulusmeasured at 160° C. to 220° C. for a separator for an electricitystorage device not containing the silane-modified polyolefin, and themeasuring conditions for the loss modulus E″ (E″_(a) or E″₀) arespecified by the following (i) to (iv):

(i) the dynamic viscoelasticity measurement is carried out under thefollowing conditions:

-   -   Measuring apparatus: RSA-G2 (TA Instruments)    -   Sample thickness: from 5 μm to 50 μm    -   Measuring temperature range: −50 to 225° C.    -   Temperature-elevating rate: 10° C./min    -   Measuring frequency: 1 Hz    -   Transform mode: sine wave tension mode (linear tension)    -   Initial static tensile load: 0.5 N    -   Initial gap distance (at 25° C.): 25 mm    -   Auto strain adjustment: Enabled (range: 0.05 to 25% amplitude,        0.02 to 5 N sine wave load);

(ii) the static tensile load is the median value of the maximum stressand minimum stress for each periodic motion, and the sine wave load isthe vibrational stress centered on the static tensile load;

(iii) the sine wave tension mode is measurement of the vibrationalstress while carrying out periodic motion at a fixed amplitude of 0.2%,wherein in sine wave tension mode, the vibrational stress is measuredwhile varying the gap distance and static tensile load so that thedifference between the static tensile load and the sine wave load iswithin 20%, and when the sine wave load is 0.02 N or lower, thevibrational stress is measured while amplifying the amplitude value sothat the sine wave load is no greater than 5 N and the increase in theamplitude value is no greater than 25%; and

(iv) the loss modulus E″ is calculated from the obtained sine wave loadand amplitude value, and the following formulas:σ*=σ₀·Exp[i(ωt+δ)],ε*=ε₀·Exp(iωt),σ*=E*·ε*E*=E′+iE″where σ*: vibrational stress, ε*: strain, i: imaginary number unit, ω:angular frequency, t: time, δ: phase difference between vibrationalstress and strain, E*: complex modulus, E″: storage modulus, E″: lossmodulus,

vibrational stress: sine wave load/initial cross-sectional area

static tensile load: load at minimum point of vibrational stress foreach period (minimum point of gap distance for each period), and

sine wave load: difference between measured vibrational stress andstatic tensile load.

[10]

The separator for an electricity storage device according to [8] or [9]above, wherein the separator for an electricity storage device notcontaining a silane-modified polyolefin is a non-silane-modifiedpolyolefin microporous membrane with a gelation degree of 0% to 10%.

[11]

A separator for an electricity storage device comprising 5 to 40 weight% of a silane-modified polyolefin and 60 to 95 weight % of a polyolefinother than the silane-modified polyolefin, wherein the transitiontemperature is 135° C. to 150° C. for the rubber plateau and the crystalmelt flow region, in the temperature-dependent change of the storagemodulus of the separator for an electricity storage device.

[12]

A separator for an electricity storage device comprising a polyolefinmicroporous membrane, wherein:

in solid viscoelasticity measurement of the separator for an electricitystorage device at a temperature of −50° C. to 250° C.,

the minimum value of the storage modulus is 1.0 MPa to 10 MPa, themaximum of the storage modulus is 100 MPa to 10,000 MPa, and

the minimum of the loss modulus is 0.1 MPa to 10 MPa and the maximum ofthe loss modulus is 10 MPa to 10,000 MPa,

the conditions for the solid viscoelasticity measurement to measure thestorage modulus and loss modulus being specified by the following (i) to(iv):

(i) the dynamic viscoelasticity measurement is carried out under thefollowing conditions:

-   -   Measuring apparatus: RSA-G2 (TA Instruments)    -   Sample thickness: 200 μm to 400 μm (with the proviso that when        the membrane thickness of the sample alone is less than 200 μm,        the dynamic viscoelasticity measurement is carried out by        stacking multiple samples so that their total thickness is 200        μm to 400 μm)    -   Measuring temperature range: −50° C. to 250° C.    -   Temperature-elevating rate: 10° C./min    -   Measuring frequency: 1 Hz    -   Transform mode: sine wave tension mode (linear tension)    -   Initial static tensile load: 0.2 N    -   Initial gap distance (at 25° C.): 10 mm    -   Auto strain adjustment: Disabled;

(ii) the static tensile load is the median value of the maximum stressand minimum stress for each periodic motion, and the sine wave load isthe vibrational stress centered on the static tensile load;

(iii) the sine wave tension mode is measurement of the vibrationalstress while carrying out periodic motion at a fixed amplitude of 0.1%,wherein in sine wave tension mode, the vibrational stress is measuredwhile varying the gap distance and static tensile load so that thedifference between the static tensile load and the sine wave load iswithin 5%, and when the sine wave load is 0.1 N or lower, thevibrational stress is measured with the static tensile load fixed at 0.1N; and

(iv) the storage modulus and loss modulus are calculated from therelationship between the obtained sine wave load and amplitude value,and the following formulas:σ=σ₀·Exp[i(ωt+δ)],ε*=ε₀·Exp(iωt),σ*=E*·ε*E*=E′+iE″where σ*: vibrational stress, ε*: strain, i: imaginary number unit, ω:angular frequency, t: time, δ: phase difference between vibrationalstress and strain, E*: complex modulus, E′: storage modulus, E″: lossmodulus,

vibrational stress: sine wave load/initial cross-sectional area

static tensile load: load at minimum point of vibrational stress foreach period (minimum point of gap distance for each period), and

sine wave load: difference between measured vibrational stress andstatic tensile load.

[13]

A separator for an electricity storage device, consisting of apolyolefin microporous membrane, wherein in solid viscoelasticitymeasurement of the separator for an electricity storage device from themembrane softening transition temperature to the membrane rupturetemperature, the mean storage modulus is 1.0 MPa to 12 MPa and the meanloss modulus is 0.5 MPa to 10 MPa.

[14]

The separator for an electricity storage device according to [13] above,wherein in the solid viscoelasticity measurement, the membrane softeningtransition temperature is 140° C. to 150° C. and the membrane rupturetemperature is 180° C. or higher.

[15]

The separator for an electricity storage device according to any one of[12] to [14] above, which comprises a silane-modified polyolefin and apolyolefin other than the silane-modified polyolefin.

[16]

The separator for an electricity storage device according to [15] above,which comprises 5 weight % to 40 weight % of the silane-modifiedpolyolefin and 60 weight % to 95 weight % of the polyolefin other thanthe silane-modified polyolefin.

[17]

A separator for an electricity storage device comprising a polyolefin,wherein:

the polyolefin has one or more types of functional groups, and

after housing the separator in the electricity storage device, (1) thefunctional groups undergo mutual condensation reaction, or (2) thefunctional groups react with chemical substances inside the electricitystorage device or (3) the functional groups react with other types offunctional groups, whereby a crosslinked structure is formed.

[18]

The separator for an electricity storage device according to [17] above,wherein the chemical substance is an electrolyte, electrolyte solution,electrode active material or additive contained in the electricitystorage device, or a decomposition product thereof.

[19]

A separator for an electricity storage device comprising a polyolefin,which has an amorphous crosslinked structure in which the amorphousportion of the polyolefin is crosslinked.

[20]

The separator for an electricity storage device according to [19] above,wherein the separator for an electricity storage device has a mixedstorage modulus ratio (R_(E′x)) of 1.5 to 20 as defined by the followingformula (1):R _(E′X) =E′ _(Z) /E′ _(Z0)  (1)where E′_(Z) is the storage modulus measured in the temperature range of160° C. to 300° C. after crosslinking reaction of the separator for anelectricity storage device has proceeded in the electricity storagedevice, and E′_(Z0) is the storage modulus measured in the temperaturerange of 160° C. to 300° C. before the separator for an electricitystorage device has been incorporated into the electricity storagedevice.[21]

The separator for an electricity storage device according to [19] or[20] above, wherein the separator for an electricity storage device hasa mixed loss modulus ratio (R_(E″x)) of 1.5 to 20 as defined by thefollowing formula (3):R _(E″X) =E″ _(Z) /E″ _(Z0)  (3)where E″_(Z) is the loss modulus measured in the temperature range of160° C. to 300° C. after crosslinking reaction of the separator for anelectricity storage device has proceeded in the electricity storagedevice, and E_(″Z0) is the loss modulus measured in the temperaturerange of 160° C. to 300° C. before the separator for an electricitystorage device has been incorporated into the electricity storagedevice.[22]

The separator for an electricity storage device according to any one of[19] to [21] above, wherein the amorphous portion is selectivelycrosslinked.

[23]

The separator for an electricity storage device according to any one of[17] to [22] above, wherein the separator for an electricity storagedevice has a mixed storage modulus ratio (R_(E′mix)) of 1.5 to 20 asdefined by the following formula (2):R _(E′mix) =E′/E′ ₀  (2)where E′ is the storage modulus measured at 160° C. to 300° C. when theseparator for an electricity storage device has an amorphous crosslinkedstructure, and E′₀ is the storage modulus measured at 160° C. to 300° C.for the separator for an electricity storage device without an amorphouscrosslinked structure.[24]

The separator for an electricity storage device according to any one of[17] to [23] above, wherein the separator for an electricity storagedevice has a mixed loss modulus ratio (R_(E″mix)) of 1.5 to 20 asdefined by the following formula (4):R _(E″mix) =E″/E″ ₀  (4)where E″ is the loss modulus measured at 160° C. to 300° C. when theseparator for an electricity storage device has an amorphous crosslinkedstructure, and E″₀ is the loss modulus measured at 160° C. to 300° C.for the separator for an electricity storage device without an amorphouscrosslinked structure.[25]

The separator for an electricity storage device according to any one of[17] to [24] above, wherein the polyolefin is polyethylene.

[26]

The separator for an electricity storage device according to any one of[17] to [25] above, wherein the polyolefin is a functionalgroup-modified polyolefin or a polyolefin obtained by copolymerizationof monomers with a functional group.

[27]

The separator for an electricity storage device according to any one of[17] to [26] above, wherein the crosslinked structure is formed byreaction via covalent bonding, hydrogen bonding or coordination bonding.

[28]

The separator for an electricity storage device according to [27] above,wherein the reaction by covalent bonding is one or more selected fromthe group consisting of the following reactions (I) to (IV):

(I) condensation reaction of a plurality of the same functional groups;

(II) reaction between a plurality of different functional groups;

(III) chain condensation reaction between a functional group and anelectrolyte solution; and

(IV) reaction between a functional group and an additive.

[29]

The separator for an electricity storage device according to [27] above,wherein the reaction by coordination bonding is the following reaction(V):

(V) reaction in which a plurality of the same functional groupscrosslink via coordination bonding with metal ions.

[30]

The separator for an electricity storage device according to [28] above,wherein reactions (I) and/or (II) are catalytically accelerated by achemical substance inside the electricity storage device.

[31]

The separator for an electricity storage device according to [28] above,wherein reaction (I) is condensation reaction of a plurality of silanolgroups.

[32]

The separator for an electricity storage device according to [28] above,wherein reaction (IV) is nucleophilic substitution reaction,nucleophilic addition reaction or ring-opening reaction between compoundRx of the separator for an electricity storage device and compound Ry ofthe additive, the compound Rx having a functional group x and thecompound Ry having a linking reaction unit y₁.

[33]

The separator for an electricity storage device according to [32] above,wherein:

reaction (IV) is nucleophilic substitution reaction,

the functional group x of compound Rx is one or more selected from thegroup consisting of —OH, —NH₂, —NH—, —COOH and —SH, and

the linking reaction unit y₁ of compound Ry consists of two or moreselected from the group consisting of CH₃SO₂—, CF₃SO₂—, ArSO₂—, CH₃SO₃—,CF₃SO₃—, ArSO₃— and monovalent groups represented by the followingformulas (y₁-1) to (y₁-6):

where X is hydrogen or a monovalent substituent,

where X is hydrogen or a monovalent substituent,

where X is hydrogen or a monovalent substituent,

where X is hydrogen or a monovalent substituent,

where X is hydrogen or a monovalent substituent, and

where X is hydrogen or a monovalent substituent.[34]

The separator for an electricity storage device according to [32] or[33] above, wherein:

reaction (IV) is nucleophilic substitution reaction,

compound Ry has a straight-chain unit y₂ in addition to the linkingreaction unit y₁, and

the straight-chain unit y₂ is one or more selected from the groupconsisting of divalent groups represented by the following formulas(y₂-1) to (y₂-6):

where m is an integer of 0 to 20, and n is an integer of 1 to 20,

where n is an integer of 1 to 20,

where n is an integer of 1 to 20,

where n is an integer of 1 to 20,

where X is an alkylene or arylene group of 1 to 20 carbon atoms, and nis an integer of 1 to 20, and

where X is an alkylene or arylene group of 1 to 20 carbon atoms, and nis an integer of 1 to 20.[35]

The separator for an electricity storage device according to [32] above,wherein:

the reaction (IV) is nucleophilic addition reaction,

the functional group x of compound Rx is one or more selected from thegroup consisting of —OH, —NH₂, —NH—, —COOH and —SH, and

the linking reaction unit y₁ of compound Ry is one or more selected fromthe group consisting of groups represented by the following formulas(Ay₁-1) to (Ay₁-6):

where R is hydrogen or a monovalent organic group,

[36]

The separator for an electricity storage device according to [32] above,wherein:

the reaction (IV) is ring-opening reaction,

the functional group x of compound Rx is one or more selected from thegroup consisting of —OH, —NH₂, —NH—, —COOH and —SH, and

the linking reaction unit y₁ of compound Ry consists of two or moregroups represented by the following formula (ROy₁-1):

where the multiple X groups are each independently a hydrogen atom or amonovalent substituent.[37]

The separator for an electricity storage device according to [29] above,wherein in reaction (V), the metal ion is one or more selected from thegroup consisting of Zn²⁺, Mn²⁺, Co³⁺, Ni²⁺ and Li⁺.

[38]

A separator for an electricity storage device comprising a first porouslayer (layer A) that includes a silane-modified polyolefin and iscapable of forming a crosslinked structure, and a second porous layer(layer B) that includes inorganic particles, wherein the heat shrinkagefactor at 150° C. after formation of the crosslinked structure is 0.02to 0.91 times the heat shrinkage factor at 150° C. before formation ofthe crosslinked structure.

[39]

The separator for an electricity storage device according to [38] above,wherein the crosslinked structure in layer A is formed by an acid, abase, swelling, or a compound generated inside the electricity storagedevice.

[40]

A separator for an electricity storage device, which comprises:

a microporous membrane that includes a silane-modified polyolefin and

an inorganic porous layer that includes inorganic particles and a resinbinder, disposed on at least one surface of the microporous membrane.

[41]

The separator for an electricity storage device according to [40] above,wherein the content of the inorganic particles in the inorganic porouslayer is 5 wt % to 99 wt %.

[42]

The separator for an electricity storage device according to [40] or[41] above, wherein the content of the silane-modified polyolefin in themicroporous membrane is 0.5 wt % to 40 wt %.

[43]

The separator for an electricity storage device according to any one of[40] to [42] above, wherein the inorganic particles are one or moreselected from the group consisting of alumina (Al₂O₃), silica, titania,zirconia, magnesia, ceria, yttria, zinc oxide, iron oxide, siliconnitride, titanium nitride, boron nitride, silicon carbide, aluminumhydroxide oxide (AlO(OH)), talc, kaolinite, dickite, nacrite,halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite,bentonite, asbestos, zeolite, diatomaceous earth, quartz sand and glassfibers.

[44]

The separator for an electricity storage device according to any one of[40] to [43] above, wherein the glass transition temperature (Tg) of theresin binder is −50° C. to 100° C.

[45]

The separator for an electricity storage device according to any one of[40] to [44], wherein silane crosslinking reaction of thesilane-modified polyolefin is initiated when the separator for anelectricity storage device contacts with an electrolyte solution.

[46]

The separator for an electricity storage device according to any one of[40] to [45], wherein the separator for an electricity storage devicehas:

a storage modulus change ratio (R_(ΔE′)) of 1.5 to 20 as defined by thefollowing formula (1A):R _(ΔE′) =E′ _(S) /E′ _(j)  (1A)where E′_(j) is the storage modulus measured at 160° C. to 220° C. forthe separator for an electricity storage device before crosslinkingreaction of the silane-modified polyolefin, and E′_(S) is the storagemodulus measured at 160° C. to 220° C. for the separator for anelectricity storage device after crosslinking reaction of thesilane-modified polyolefin, and/or

a loss modulus change ratio (R_(ΔE″)) of 1.5 to 20 as defined by thefollowing formula (1B):R _(ΔE″) =E″ _(S) /E″ _(j)  (1B)where E″_(j) is the loss modulus measured at 160° C. to 220° C. for theseparator for an electricity storage device before crosslinking reactionof the silane-modified polyolefin, and E″_(S) is the loss modulusmeasured at 160° C. to 220° C. for the separator for an electricitystorage device after crosslinking reaction of the silane-modifiedpolyolefin,when measured after the inorganic porous layer has been removed.[47]

The separator for an electricity storage device according to any one of[40] to [46], wherein the separator for an electricity storage devicehas:

a mixed storage modulus ratio (R_(E′mix)) of 1.5 to 20 as defined by thefollowing formula (2A):R _(E′mix) =E′/E′ ₀  (2A)where E′ is the storage modulus measured at 160° C. to 220° C. for theseparator for an electricity storage device and E′₀ is the storagemodulus measured at 160° C. to 220° C. for a separator for anelectricity storage device not containing the silane-modifiedpolyolefin, and/or

a mixed loss modulus ratio (R_(E″mix)) of 1.5 to 20 as defined by thefollowing formula (2B):R _(E″mix) =E″/E″ ₀  (2B)where E″ is the loss modulus measured at 160° C. to 220° C. for theseparator for an electricity storage device, and E″₀ is the loss modulusmeasured at 160° C. to 220° C. for as separator for an electricitystorage device not containing the silane-modified polyolefin, whenmeasured after the inorganic porous layer has been removed.[48]

The separator for an electricity storage device according to any one of[40] to [47] above, wherein, in the temperature-dependent change of thestorage modulus of the separator for an electricity storage device, thetransition temperature of the rubber plateau and the crystal melt flowregion is 135° C. to 150° C.

[49]

An electricity storage device comprising an electrode, the separator foran electricity storage device according to any one of [1] to [48] above,and a nonaqueous electrolyte solution.

[50]

An electricity storage device comprising a separator that includespolyethylene, and an electrolyte solution or additive, wherein afunctional group-modified polyethylene or functional group graftcopolymerized polyethylene reacts with a chemical substance in theelectrolyte solution or additive, forming a crosslinked structure.

[51]

A method for producing the separator for an electricity storage deviceaccording to any one of [1] to [50] above, wherein the method comprisesthe following steps:

(1) a sheet-forming step in which a mixture of a silane-modifiedpolyolefin, polyethylene and a plasticizer is extruded, cooled tosolidification and cast into a sheet to obtain a sheet;

(2) a stretching step in which the sheet is stretched at least in auniaxial direction to obtain a stretched sheet;

(3) a porous body-forming step in which the plasticizer is extractedfrom the stretched sheet in the presence of an extraction solvent,forming pores in the stretched sheet to form a porous body; and

(4) a heat treatment step in which the porous body is subjected to heattreatment.

[52]

A method for producing a separator for an electricity storage device,which comprises the following steps:

(1) a sheet-forming step in which a silane-modified polyolefin,polyethylene and a plasticizer are extruded into a sheet using anextruder, cooled to solidification and shaped into a molded sheet;

(2) a stretching step in which the molded sheet is subjected to biaxialstretching to a 20-fold to 250-fold area increase to form a stretchedsheet;

(3) a porous body-forming step in which the plasticizer is extractedfrom the stretched sheet to form a porous body;

(4) a heat treatment step in which the porous body is subjected to heattreatment and subjected to stretching and relaxation in the transversedirection to obtain a heat-treated porous body;

(8B) a coating step in which an inorganic porous layer includinginorganic particles and a resin binder is formed on at least one surfaceof the heat-treated porous body to form a silane-crosslinking precursor;and

(9) an assembly step in which a laminated stack or wound body ofelectrodes and the silane-crosslinking precursor, and a nonaqueouselectrolyte solution, are housed in an exterior body, contacting thesilane-crosslinking precursor with the nonaqueous electrolyte solution.

[53]

An electricity storage device assembly kit comprising the following twoelements:

(1) an exterior body housing a laminated stack or wound body ofelectrodes and the separator for an electricity storage device accordingto any one of [1] to [48] above; and

(2) a container housing a nonaqueous electrolyte solution.

[54]

The electricity storage device assembly kit according to [53] above,wherein the nonaqueous electrolyte solution includes a fluorine(F)-containing lithium salt.

[55]

The electricity storage device assembly kit according to [53] or [54],wherein the nonaqueous electrolyte solution includes lithiumhexafluorophosphate (LiPF₆).

[56]

The electricity storage device assembly kit according to any one of [53]to [55], wherein the nonaqueous electrolyte solution is an acid solutionand/or a base solution.

[57] A method for producing an electricity storage device comprising thefollowing steps:

a step of preparing the electricity storage device assembly kitaccording to any one of [53] to [56] above, and

a step of contacting the separator for an electricity storage device inelement (1) of the electricity storage device assembly kit with thenonaqueous electrolyte solution in element (2), to initiate silanecrosslinking reaction of the silane-modified polyolefin.

[58]

The method for producing an electricity storage device according to[57], which further comprises the following steps:

a step of connecting lead terminals to the electrodes of element (1),and

a step of carrying out at least one cycle of charge-discharge.

[59]

A method for producing an electricity storage device using a separatorthat includes a polyolefin, wherein:

the polyolefin comprises one functional group or two or more differentfunctional groups, and the method for producing an electricity storagedevice comprises the following step:

a crosslinking step in which (1) condensation reaction is carried outbetween the functional groups, (2) the functional groups are reactedwith a chemical substance inside the electricity storage device, or (3)the functional groups are reacted with different types of functionalgroups, to form a crosslinked structure.

[60]

The method for producing an electricity storage device according to [59]above, wherein the crosslinking step is carried out at a temperature of5° C. to 90° C.

Advantageous Effects of Invention

According to the invention it is possible to provide an electricitystorage device which has a separator for an electricity storage devicewith both a low temperature shutdown function and high-temperaturemembrane rupture resistance, and inhibited generation of non-moltenresin aggregates during the production process, thus contributing toproductivity and economy, while also having satisfactory cyclecharacteristics and high safety, as well as an assembly kit for thesame.

Moreover, since it is not necessary to form the crosslinked structureduring the membrane formation process or immediately afterwards for thepresent invention, it is also possible to inhibit increase in internalstress of the separator or its deformation immediately after fabricationof the electricity storage device, and/or to impart a crosslinkedstructure to the separator without using the relatively high energy ofphotoirradiation or heating, and to thus reduce crosslinking unevenness.Furthermore, according to the invention it is possible to form acrosslinked structure not only within the separator but also between theseparator and the electrodes and between the separator and the solidelectrolyte interface (SEI), thus improving the strength between thedifferent members of the electricity storage device, and also to reducegaps formed between the electrodes and the separator caused by theirexpansion/contraction during charge-discharge of the electricity storagedevice, thereby notably improving cycle stability during prolonged use.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an example of a graph illustrating the relationship betweentemperature and storage modulus, contrasting the storage modulus of areference membrane and a crosslinked membrane in a temperature range of−50° C. to 225° C., and showing the transition temperature of the rubberplateau and crystal melt flow region.

FIG. 2 shows an example of a graph illustrating the relationship betweentemperature and loss modulus, contrasting the loss modulus of areference membrane and a crosslinked membrane in a temperature range of−50° C. to 225° C., and showing the transition temperature of the rubberplateau and crystal melt flow region.

FIG. 3 is a graph showing the relationship between temperature andresistance for an electricity storage device comprising the separatorobtained in Example I-1.

FIG. 4 is a graph illustrating the relationship between the temperature,gap distance, storage modulus and loss modulus in viscoelasticitymeasurement of a separator for an electricity storage device, with graph(a) for Example II-1 and graph (b) for Comparative Example II-1.

FIG. 5 is a graph for determining the membrane softening transitiontemperature in viscoelasticity measurement of a separator for anelectricity storage device, based on temperature, gap distance and firstderivative of gap displacement, with graph (a) for Example II-1 andgraph (b) for Comparative Example II-1.

FIG. 6 is a schematic diagram illustrating a crystalline polymer with ahigher-order structure, with the crystal structure divided into thelamella (crystal portion), the amorphous portion and the interlayerportion between them.

FIG. 7 is a schematic diagram illustrating crystal growth of apolyolefin molecule.

FIG. 8 is a strain-crystal fragmentation rate graph showing the changein X-ray crystal structure during a tensile rupture fracture test, for amembrane according to one embodiment of the invention.

FIG. 9 shows an example of a graph illustrating the relationship betweentemperature and storage modulus, contrasting the storage modulus of areference membrane and a crosslinked membrane in a temperature range of−50° C. to 310° C., and showing the transition temperature of the rubberplateau and crystal melt flow region.

FIG. 10 shows an example of a graph illustrating the relationshipbetween temperature and loss modulus, contrasting the loss modulus of areference membrane and a crosslinked membrane in a temperature range of−50° C. to 310° C., and showing the transition temperature of the rubberplateau and crystal melt flow region.

FIG. 11 is a ¹H-NMR chart (a) and ¹³C-NMR chart (b) for silane-modifiedpolyolefin starting material 1 obtained using a polyolefin.

FIG. 12 is a ¹H-NMR chart (a) and ¹³C-NMR chart (b) for silane-modifiedpolyolefin starting material 2 obtained using a polyolefin.

FIG. 13 is a ¹H-NMR chart (a) and ¹³C-NMR chart (b) for the separatorobtained in Example I-1, in the state before crosslinking.

DESCRIPTION OF EMBODIMENTS

Embodiments for carrying out the invention (hereunder referred to as“embodiments”) will now be explained in detail. The present invention isnot limited to the embodiments described below, and variousmodifications may be implemented within the scope of the gist thereof.

Throughout the present specification, the “to” in a numerical rangemeans that the numerical values on either side are included as the upperlimit and lower limit. The upper limits and lower limits for thenumerical ranges throughout the present specification may be combined asdesired. For example, the upper limit of a preferred numerical range maybe combined with the lower limit of a more preferred numerical range, orconversely, the upper limit of a more preferred numerical range may becombined with the lower limit of a preferred range.

Throughout the present specification, “above”, “upper” and “formed onthe side” do not mean that the positional relationship of the respectivemembers is “directly above”. For example, the expressions “layer Bformed on layer A” and “layer B formed on the surface of layer A” do notexclude the case where an arbitrary layer not qualifying as either isincluded between layer A and layer B.

The properties of the microporous membrane alone that are describedbelow may be measured after removing layers other than the microporousmembrane (for example, an inorganic porous layer) from the separator foran electricity storage device.

<Separator for Electricity Storage Device>

One aspect of the invention is a separator for an electricity storagedevice (hereunder also referred to simply as “separator”). Theseparator, which must have an insulating property and ion permeability,will usually comprise an insulating material sheet with a porous bodystructure, a polyolefin nonwoven fabric or a resin microporous membrane.Particularly suited for a lithium ion battery is a polyolefinmicroporous membrane that allows construction of a compact homogeneousporous body structure with redox degradation resistance of theseparator.

A microporous membrane is a membrane composed of a porous body, and itsmean pore size is preferably 10 nm to 500 nm and more preferably 30 nmto 100 nm.

When the separator is included in an electricity storage device, theseparator can be removed from the electricity storage device.

First, Second, Third, Fourth and Fifth Embodiments

The separator of the first embodiment comprises a silane-modifiedpolyolefin, and optionally another polyolefin as well. When theseparator of the first embodiment contacts with the electrolytesolution, silane crosslinking reaction of the silane-modified polyolefinin the separator is initiated. Since the separator of the firstembodiment is able to crosslink the silane-modified polyolefin duringcontact with the electrolyte solution, it allows timing of crosslinkingto be controlled so that crosslinking reaction can be carried out duringthe production process for the electricity storage device, instead ofcrosslinking reaction during the separator production process.

It is a feature of the separator of the second embodiment that silanecrosslinking reaction of the silane-modified polyolefin takes place whenit contacts with the electrolyte solution. According to the secondembodiment, it is sufficient for silane crosslinking reaction to beobserved during contact between the separator and electrolyte solution,regardless of whether or not the silane-modified polyolefin is presentin the separator, or where the residual silane-modified polyolefin is,or whether the silane crosslinking reaction is first initiated atcontact with the electrolyte solution, or whether it takes placesequentially or continuously. The silane crosslinking reaction of thesilane-modified polyolefin that takes place during contact between theseparator and electrolyte solution according to the second embodimentallows the timing of crosslinking to be controlled without depending onthe process of production or use of the separator.

Since the separator of the first and second embodiments can acceleratecrosslinking reaction when the electrolyte solution is injected into theexterior body that is to house the separator, it is possible to avoidproduction defects during its production process and to increase thesafety and output of the electricity storage device during theproduction process for the electricity storage device. From theviewpoint of timing of crosslinking reaction with the components of theseparator, it is preferred for the silane crosslinking reaction of thesilane-modified polyolefin to be initiated at the time of mixing orcontact between the separator and electrolyte solution.

The separator according to the third embodiment comprises 5 to 40 weight% of a silane-modified polyolefin and 60 to 95 weight % of a polyolefinother than the silane-modified polyolefin, and in the viscoelasticitymeasurement (version 1) described in the Examples, it has:

a storage modulus change ratio (RAE) of 1.5 to 20 as defined by thefollowing formula (1):R _(ΔE′) =E′ _(S) /E′ _(j)  (1){where E′_(j) is the storage modulus measured at 160° C. to 220° C. forthe separator for an electricity storage device before crosslinkingreaction of the silane-modified polyolefin, and E′_(S) is the storagemodulus measured at 160° C. to 220° C. for the separator for anelectricity storage device after crosslinking reaction of thesilane-modified polyolefin}, and/or

a loss modulus change ratio (R_(ΔE″)) of 1.5 to 20 as defined by thefollowing formula (3):R _(ΔE″) =E″ _(S) /E″ _(j)  (3){where E″_(j) is the loss modulus measured at 160° C. to 220° C. for theseparator for an electricity storage device before crosslinking reactionof the silane-modified polyolefin, and E″_(S) is the loss modulusmeasured at 160° C. to 220° C. for the separator for an electricitystorage device after crosslinking reaction of the silane-modifiedpolyolefin}. Since the storage modulus change ratio (R_(ΔE′)) and/or theloss modulus change ratio (R_(ΔE″)) are within the range of 1.5 to 20according to the third embodiment, it is possible to obtain both ashutdown function and high-temperature membrane rupture resistance. Thestorage modulus change ratio (R_(ΔE′)) and/or loss modulus change ratio(R_(ΔE″)) are preferably between 2 and 18. Incidentally, E′_(j) andE′_(S), and E″_(j) and E″_(S), are the average values for the storagemodulus or loss modulus measured in the preset temperature range of themeasuring apparatus, with 160 to 220° C. set as the widest temperaturerange for each. When the separator is in the form of a laminatedmembrane, the silane-modified polyolefin-containing porous membranealone is removed from the laminated membrane and the storage modulusE′_(j) and E′_(S) and the loss modulus E″_(j) and E″_(S) are measured.

The separator according to the fourth embodiment comprises 5 to 40weight % of a silane-modified polyolefin and 60 to 95 weight % of apolyolefin other than the silane-modified polyolefin, and in theviscoelasticity measurement (version 1) described in the Examples, ithas:

a mixed storage modulus ratio (R_(E′mix)) of 1.5 to 20 as defined by thefollowing formula (2):R _(E′mix) =E′ _(a) /E′ ₀  (2){where E′_(a) is the storage modulus measured at 160° C. to 220° C. forthe separator for an electricity storage device, and E′₀ is the storagemodulus measured at 160° C. to 220° C. for a separator for anelectricity storage device not containing the silane-modifiedpolyolefin}, and/or

a mixed loss modulus ratio (R_(E″mix)) of 1.5 to 20.0 as defined by thefollowing formula (4):R _(E″mix) =E″ _(a) /E″ ₀  (4){where E″_(a) is the loss modulus measured at 160° C. to 220° C. for theseparator for an electricity storage device, and E″₀ is the loss modulusmeasured at 160° C. to 220° C. for a separator for an electricitystorage device not containing the silane-modified polyolefin}. Since themixed storage modulus ratio (R_(E′mix)) and/or the mixed loss modulusratio (R_(E″mix)) are within the range of 1.5 to 20.0 according to thefourth embodiment, it is possible to obtain both a shutdown function andhigh-temperature membrane rupture resistance. The mixed storage modulusratio (R_(E′mix)) and/or mixed loss modulus ratio (R_(E″mix)) arepreferably between 2 and 18. Incidentally, E′_(a) and E′₀, and E″_(a)and E″₀, are the average values for the storage modulus or loss modulusmeasured in the preset temperature range of the measuring apparatus,with 160 to 220° C. set as the widest temperature range for each. Whenthe separator is in the form of a laminated membrane, thesilane-modified polyolefin-containing porous membrane alone is removedfrom the laminated membrane and the storage modulus E′_(a) and E′₀ andthe loss modulus E″_(a) and E″₀ are measured.

The separator of the fifth embodiment comprises 5 to 40 weight % of asilane-modified polyolefin and 60 to 95 weight % of a polyolefin otherthan the silane-modified polyolefin, wherein the transition temperatureis 135° C. to 150° C. for the rubber plateau and the crystal melt flowregion for the temperature-dependent change of the storage modulus orloss modulus, in the viscoelasticity measurement (version 1) describedin the Examples. Since the transition temperature of the rubber plateauand crystal melt flow region is in the range of 135° C. to 150° C.according to the fifth embodiment, it is possible to obtain both ashutdown function and high-temperature membrane rupture resistance. Thetransition temperature of the rubber plateau and crystal melt flowregion is preferably 137° C. to 147° C., more preferably 140° C. to 145°C. and even more preferably 140° C. to 143° C. When the separator is inthe form of a laminated membrane, the silane-modifiedpolyolefin-containing porous membrane alone is removed from thelaminated stack and the transition temperature of the rubber plateau andcrystal melt flow region is measured.

Sixth and Seventh Embodiments

The separator according to the sixth embodiment comprises a polyolefinhaving one or more types of functional groups, and after being housed inan electricity storage device, (1) the functional groups of thepolyolefin undergo mutual condensation reaction, or (2) the functionalgroups of the polyolefin react with chemical substances inside theelectricity storage device or (3) the functional groups of thepolyolefin react with other types of functional groups, thus forming acrosslinked structure. Since it is thought that the functional groups inthe polyolefin composing the separator are not incorporated into thecrystal portion of the polyolefin but are crosslinked at the amorphousportion, then after the separator of the sixth embodiment has beenhoused in an electricity storage device, chemical substances in thesurrounding environment or inside the electricity storage device can beutilized to form a crosslinked structure, thereby inhibiting increase ininternal stress or deformation of the fabricated electricity storagedevice.

However, when crosslinking reaction is carried out before housing in theelectricity storage device and steps such as winding up and slitting arecarried out, this leaves effects of the stress produced by the tensileforce generated during the steps. This is undesirable because when suchstress is released after assembly of the electricity storage device itcan potentially damage the wound electrodes by deformation orconcentration of stress.

According to the sixth embodiment, the condensation reaction between thefunctional groups of the polyolefin (1) may be reaction by covalentbonding of two or more functional groups A in the polyolefin, forexample. The reaction between the functional groups of the polyolefinand other types of functional groups (3) may be reaction by covalentbonding of a functional group A and a functional group B in thepolyolefin, for example.

In the reaction between functional groups of the polyolefin and chemicalsubstances inside the electricity storage device (2), for example, afunctional group A in the polyolefin may form covalent bonds orcoordination bonds with the electrolyte, electrolyte solution, electrodeactive material, electrode active material, additive or theirdecomposition products in the electricity storage device. In reaction(2), a crosslinked structure may be formed not only within the separatorbut also between the separator and the electrodes or between theseparator and the solid electrolyte interface (SEI), thus increasing thestrength between multiple members of the electricity storage device.

The separator of the seventh embodiment comprises a polyolefin and hasan amorphous crosslinked structure in which the amorphous portion of thepolyolefin is crosslinked. Since it is believed that the functionalgroups in the polyolefin composing the separator are not incorporatedinto the crystal portion of the polyolefin but are instead crosslinkedin the amorphous portion, the separator of the seventh embodiment canprovide both a shutdown function and high-temperature membrane ruptureresistance while inhibiting increase in internal stress or deformationof the fabricated electricity storage device, as compared to aconventional crosslinked separator in which the crystal portion and itsperiphery are easily crosslinked, and can therefore ensure the safety ofthe electricity storage device. From the same viewpoint, the amorphousportion of the polyolefin in the separator of the seventh embodiment ispreferably selectively crosslinked, and more preferably it issignificantly more crosslinked than the crystal portion.

The crosslinking reaction mechanism and crosslinked structure of theseventh embodiment are not fully understood, but are conjectured by thepresent inventors to be as follows.

(1) Crystal Structure of High-Density Polyethylene Microporous Membrane

A polyolefin resin, which is typically high-density polyethylene, isgenerally a crystalline polymer, and as shown in FIG. 6 , it has ahigher-order structure divided into the lamella of the crystal structure(crystal portion), an amorphous portion and an interlayer portionbetween them. The polymer chain mobility is low in the crystal portionand in the interlayer portion between the crystal portion and amorphousportion, making it difficult to separate, but in solid viscoelasticitymeasurement it is possible to observe a relaxation phenomenon in therange of 0 to 120° C. The amorphous portion, on the other hand, has veryhigh polymer chain mobility, with the phenomenon being observed in therange of −150 to −100° C. in solid viscoelasticity measurement. This isclosely related to the radical relaxation or radical transfer reactionor crosslinking reaction described below.

Moreover, the polyolefin molecules composing the crystals are not simplebut rather, as shown in FIG. 7 , multiple polymer chains form smalllamella which then aggregate, forming crystals. It is difficult toobserve this phenomenon directly. It has become evident, however, asrecent simulations have advanced academic research in the field. For thepurpose described herein, a “crystal” is the minimum crystal unitmeasured by X-ray structural analysis, being a unit that can becalculated as crystallite size. Thus, even the crystal portion (lamellainterior) is partially unconstrained during crystallization, so thatportions with somewhat high mobility are predicted to be present.

(2) Crosslinking Reaction Mechanism by Electron Beam

The reaction mechanism of electron beam crosslinking (hereunderabbreviated as EB crosslinking) of polymers is as follows. (I)Irradiation of an electron beam of several tens of kGy to severalhundred kGy, (ii) permeation of the electron beam into the reactiontarget (polymer) and secondary electron generation, (iii) hydrogenwithdrawal reaction and radical generation in the polymer chains by thesecondary electrons, (iv) withdrawal of adjacent hydrogens by radicalsand migration of the active sites, and (v) crosslinking reaction orpolyene formation by recombination between radicals. Because radicalsgenerated in the crystal portion have poor mobility they are present forlong periods, while impurities and the like are unable to infiltrateinto the crystals, and therefore the probability of reaction orquenching is low. Such radical species are known as stable radicals, andthey remain for long periods of several months, their lifetimes havingbeen elucidated by ESR measurement. This is thought to result in poorcrosslinking reaction within the crystals. However, in the unconstrainedmolecular chains or the peripheral crystal-amorphous interlayer portionswhich are present in small amounts inside the crystals, the generatedradicals have somewhat longer lifetimes. These radical species are knownas persistent radicals, and in mobile environments they are thought topromote crosslinking reaction between molecular chains with highprobability. The amorphous portions have very high mobility, andtherefore generated radical species have a short lifetime and arethought to promote not only crosslinking reaction between molecularchains but also polyene reaction within individual molecular chains,with high probability.

In a micro visual field on the level of crystals, therefore,crosslinking reaction by EB crosslinking can be assumed to be localizedwithin the crystals or at their peripheries.

(3) Crosslinking Reaction Mechanism by Chemical Reaction

According to the seventh embodiment of the invention, the functionalgroups in the polyolefin resin and the chemical substances in theelectricity storage device, or the chemical substances in theelectricity storage device, are preferably used as catalysts.

As mentioned above, crystal portions and amorphous portions are presentin a polyolefin resin. Due to steric hindrance however, the functionalgroups are not present in the crystals and are localized in theamorphous portions. This is generally known, and even though units suchas methyl groups that are present in small amounts in polyethylenechains are incorporated into the crystals, grafts that are bulkier thanethyl groups are not incorporated (NPL 2). Therefore, crosslinkingpoints due to different reactions than the electron beam crosslinkingare only localized at the amorphous portions.

(4) Relationship Between Differences in Crosslinked Structure and theirEffects

As mentioned above, the reaction products in the crosslinking reactionsby chemical reactions within the battery used for the seventh embodimentof the invention have different morphologies. In the research leading tothe present invention, the following experimentation was carried out inorder to elucidate crosslinked structures and to characterize thechanges in physical properties of microporous membranes that result fromstructural changes.

First, the mechanical properties of a membrane were examined by atensile rupture test. Simultaneously with the tensile rupture test,in-situ X-ray structural analysis was carried out using emitted light toanalyze changes in crystal structure. As shown in FIG. 8 , compared to amembrane without EB crosslinking or chemical crosslinking (before), theEB crosslinked membrane had reduced fragmentation of the crystal portionas the strain increased. This is because the crystal interiors orperipheries had been selectively crosslinked. The Young's modulus andbreaking strength markedly increased during this time, allowing highmechanical strength to be exhibited. The chemical crosslinked membrane,on the other hand, showed no difference in fragmentation of the crystalsbefore and after crosslinking reaction, thus suggesting that theamorphous portion has been selectively crosslinked. There was also nochange in mechanical strength before and after crosslinking reaction.

The crystal melt behavior of both was then examined in a fuse/meltdownproperty test. As a result, the EB crosslinked membrane had a notablyhigher fuse temperature, and the meltdown temperature increased to 200°C. or higher. The chemical crosslinked membrane, on the other hand,showed no change in fuse temperature before and after crosslinkingtreatment, and the meltdown temperature was confirmed to have increasedto 200° C. or higher. This suggests that the fuse (shutdown) propertiesresulting from crystal melting had resulted from a higher meltingtemperature and lower melting speed due to crosslinking of the EBcrosslinked membrane at the peripheries of the crystal portions. It wasalso concluded that no change was caused in the shutdown propertybecause the chemical crosslinked membrane had no crosslinked structureat the crystal portions. In the high temperature range of around 200°C., both had a crosslinked structure after crystal melting, andtherefore the resin as a whole was stabilized in a gel state and asatisfactory meltdown property was obtained.

These findings are summarized in the following table.

TABLE 1 Electron beam Chemical reactive crosslinking crosslinkingCrosslinking Within crystals and Amorphous site at crystal- portionsamorphous interlayer portions Film strength Increased No change Fusefunction Function impaired or lost No change Meltdown Gradual increasewith dose Definitely resistance improved

From the viewpoint of formation of an amorphous crosslinked structure,and obtaining both a shutdown function and high-temperature membranerupture resistance, the separator of the seventh embodiment has, in theviscoelasticity measurement (version 2) described in the Examples:

a mixed storage modulus ratio (R_(E′x)) defined by the following formula(1):R _(E′X) =E′ _(Z) /E′ _(z0)  (1){where E′_(Z) is the storage modulus measured in the temperature rangeof 160° C. to 300° C. after crosslinking reaction of the separator foran electricity storage device has proceeded in the electricity storagedevice, and

E′_(z0) is the storage modulus measured in the temperature range of 160°C. to 300° C. before the separator for an electricity storage device hasbeen incorporated into the electricity storage device}, and/or

a mixed loss modulus ratio (R_(E″x)) of preferably 1.5 to 20 and morepreferably 3 to 18, defined by the following formula (3):R _(E″X) =E″ _(Z) /E″ _(Z0)  (3){where E″_(Z) is the loss modulus measured in the temperature range of160° C. to 300° C. after crosslinking reaction of the separator for anelectricity storage device has proceeded in the electricity storagedevice, and

E″_(Z0) is the loss modulus measured in the temperature range of 160° C.to 300° C. before the separator for an electricity storage device hasbeen incorporated into the electricity storage device}. Incidentally,E′_(Z), E′_(z0) and E″_(Z), E″_(z0) are the average values for thestorage modulus or loss modulus measured in the preset temperature rangeof the measuring apparatus, with 160° C. to 300° C. set as the widesttemperature range for each. When the separator is in the form of alaminated membrane, the polyolefin porous membrane alone is removed fromthe laminated membrane and the storage modulus E′_(Z) and E′_(z0) andthe loss modulus E″_(Z) and E″_(z0) are measured.

From the viewpoint of formation of an amorphous crosslinked structure,and obtaining both a shutdown function and high-temperature membranerupture resistance, the separators according to the sixth and seventhembodiments have, in the viscoelasticity measurement (version 2)described in the Examples:

-   -   a mixed storage modulus ratio (R_(E′mix)) defined by the        following formula (2):        R _(E′mix) =E′/E′ ₀  (2)        {where E′ is the storage modulus measured in the temperature        range of 160° C. to 300° C. for the separator for an electricity        storage device having an amorphous crosslinked structure, and

E′₀ is the storage modulus measured at 160° C. to 300° C. for theseparator for an electricity storage device without an amorphouscrosslinked structure}, and/or

a mixed loss modulus ratio (R_(E″mix)) of preferably 1.5 to 20, morepreferably 3 to 19 and even more preferably 5 to 18, defined by thefollowing formula (4):R _(E″mix) =E″/E″ ₀  (4){where E″ is the loss modulus measured at 160° C. to 300° C. when theseparator for an electricity storage device has an amorphous crosslinkedstructure, and

E″₀ is the loss modulus measured at 160° C. to 300° C. for the separatorfor an electricity storage device without an amorphous crosslinkedstructure}. Incidentally, E′, E′₀ and E″, E″₀ are the average values forthe storage modulus or loss modulus measured in the preset temperaturerange of the measuring apparatus, with 160° C. to 300° C. set as thewidest temperature range for each. When the separator is in the form ofa laminated membrane, the polyolefin porous membrane alone is removedfrom the laminated membrane and the storage modulus E′ and E′₀ and theloss modulus E″ and E″₀ are measured.

Eighth Embodiment

[Viscoelastic Behavior (Viscoelasticity Measurement (Version 3)Described in the Examples)]

The separator according to the eighth embodiment comprises a polyolefinmicroporous membrane, and in viscoelasticity measurement (version 3)described in the Examples, by solid viscoelasticity measurement at atemperature of −50° C. to 250° C., the minimum of the storage modulus(E′) (E′_(min)) is 1.0 MPa to 10 MPa, the maximum of E′ (E′_(max)) is100 MPa to 10,000 MPa, and/or the minimum of the loss modulus (E″)(E″_(min)) is 0.1 MPa to 10 MPa and the maximum of E″ (E″_(max)) is 10MPa to 10,000 MPa. If the ranges of 1.0 MPa≤E′_(min)≤10 MPa and 100MPa≤E′_(max)≤10,000 MPa and/or 0.1 MPa≤E″_(min)≤10 MPa and 10MPa≤E″_(max)≤10,000 MPa are satisfied, then not only will the separatortend to have both a shutdown function and high-temperature membranerupture resistance, but production defects can be avoided during theproduction process for the separator or electricity storage device, andstability and safety can be achieved for the electricity storage device.From this viewpoint, the ranges are preferably 1.1 MPa≤E′_(min)≤9.0 MPaand/or 150 MPa≤E′_(max)≤9,500 MPa, and more preferably 1.2MPa≤E′_(min)≤8.0 MPa and/or 233 MPa≤E′_(max)≤9,000 MPa. Also, the ranges0.2 MPa≤E″_(min)≤9.0 MPa and/or 56 MPa≤E″_(max)≤9,000 MPa are preferred,and 0.4 MPa≤E″_(min)≤8.0 MPa and/or 74 MPa≤E″_(max)≤8,000 MPa are morepreferred.

In solid viscoelasticity measurement (version 3) at temperatures fromthe membrane softening transition temperature to the membrane rupturetemperature of the separator comprising a polyolefin microporousmembrane, the average E′ (E′_(ave)) is preferably 1.0 MPa to 12 MPa,more preferably 1.2 MPa to 10 MPa and even more preferably 1.8 MPa to8.2 MPa, and/or the average E″ (E″_(ave)) is preferably 0.5 MPa to 10MPa, more preferably 0.8 MPa to 8.2 MPa or 0.9 MPa to 3.2 MPa. If E′and/or E″ are within these numerical ranges at temperatures from themembrane softening transition temperature to the membrane rupturetemperature, the cycle stability and safety of an electricity storagedevice comprising the separator will tend to be improved.

In solid viscoelasticity measurement (version 3), from the viewpoint ofboth the shutdown function and high-temperature membrane ruptureresistance, the membrane softening transition temperature of theseparator comprising a polyolefin microporous membrane is preferably140° C. to 150° C., more preferably 141° C. to 149° C. or 146° C. to149° C., and/or the membrane rupture temperature is preferably 180° C.or higher, more preferably 190° C. or higher, 200° C. or higher, 210° C.or higher, 220° C. or higher, 230° C. or higher or 240° C. or higher,and even more preferably 250° C. or higher. The upper limit for themembrane rupture temperature is not limited, but it is understood in thetechnical field that the same membrane rupture phenomenon may occur evenat temperatures higher than 250° C.

The conditions for measuring E′ and E″ in solid viscoelasticitymeasurement (version 3) of the separator are described in the Examples.When the separator is in the form of a laminated membrane, thepolyolefin microporous membrane alone is removed from the laminatedmembrane and the E′ and E″ values of the removed polyolefin microporousmembrane are measured. Moreover, when the membrane thickness of thepolyolefin microporous membrane alone is less than 200 μm, the dynamicviscoelasticity measurement (version 3) may be carried out with multiplepolyolefin microporous membranes stacked, or a single polyolefinmicroporous membrane folded, so that their total thickness is in therange of 200 μm to 400 μm.

From the viewpoint of both a shutdown function at relatively lowtemperature and membrane rupture properties at relatively hightemperature, as well as improved cycle characteristics and safety of theelectricity storage device, the separators of the first to eighthembodiments may also comprise a microporous membrane; and an inorganicporous layer including inorganic particles and a resin binder, disposedon at least one surface of the microporous membrane. The separator mayalso employ the microporous membrane as a base material, and may consistof a composite of the base material and an inorganic coating layer.

Ninth Embodiment

The separator according to the ninth embodiment comprises:

a microporous membrane that includes a silane-modified polyolefin and

an inorganic porous layer that includes inorganic particles and a resinbinder, disposed on at least one surface of the microporous membrane.The separator of the ninth embodiment may also optionally include alayer other than the microporous membrane and inorganic porous layer.

For the ninth embodiment, a combination of a silane-modifiedpolyolefin-containing microporous membrane and an inorganic porous layerwill tend to provide both a shutdown function at lower temperatures than150° C. and membrane rupture properties at relatively high temperature,and to improve the electricity storage device cycle characteristics andbattery nail penetration safety. Since the silane-modified polyolefin inthe microporous membrane has a silane crosslinking property, presumablysilane crosslinking can result in increased viscosity of the resin inthe microporous membrane, and therefore when compressive force isapplied between the electrodes during a period of abnormal hightemperature of an electricity storage device comprising a separator ofthe ninth embodiment, the crosslinked high-viscosity resin is lesslikely to flow into the inorganic layer (that is, integration is lesslikely), and the clearance between the electrodes can be adequatelyensured and shorting of the battery can be inhibited.

Silane crosslinking reaction of the silane-modified polyolefin in theseparator is preferably initiated when the separator of the ninthembodiment contacts with the electrolyte solution. More preferably,silane crosslinking reaction is observed during contact between theseparator and electrolyte solution, regardless of whether silanecrosslinking reaction is first initiated at contact with the electrolytesolution, or takes place sequentially or continuously. The silanecrosslinking reaction of the silane-modified polyolefin that takes placeduring contact between the separator and the electrolyte solutioncontrols the timing of crosslinking of the separator, not only avoidingproduction defects during the separator production process but alsoallowing safety and high output to be achieved in the production processfor the electricity storage device. Moreover, causing contact betweenthe separator and electrolyte solution can produce crosslinkingreactions other than the silane crosslinking reaction.

The separator according to the ninth embodiment has:

-   -   a storage modulus change ratio (RAE) of preferably 1.5 to 20 as        defined by the following formula (1A):        R _(ΔE′) =E′ _(S) /E′ _(j)  (1A)        {where E′_(j) is the storage modulus measured at 160° C. to        220° C. for the separator for an electricity storage device        before crosslinking reaction of the silane-modified polyolefin,        and E′_(S) is the storage modulus measured at 160° C. to 220° C.        for the separator for an electricity storage device after        crosslinking reaction of the silane-modified polyolefin}, and/or

a loss modulus change ratio (R_(ΔE″)) of preferably 1.5 to 20 as definedby the following formula (1B):R _(ΔE″) =E″ _(S) /E″ _(j)  (1B){where E″_(j) is the loss modulus measured at 160° C. to 220° C. for theseparator for an electricity storage device before crosslinking reactionof the silane-modified polyolefin, and E″_(S) is the loss modulusmeasured at 160° C. to 220° C. for the separator for an electricitystorage device after crosslinking reaction of the silane-modifiedpolyolefin},when measured after the inorganic porous layer has been removed from theseparator, in the viscoelasticity measurement (version 1) described inthe Examples. Since the storage modulus change ratio (R_(ΔE′)) and/orthe loss modulus change ratio (R_(ΔE″)) are within the range of 1.5 to20, it is easy to obtain both a shutdown function and high-temperaturemembrane rupture resistance. The storage modulus change ratio (RAE)and/or loss modulus change ratio (R_(ΔE″)) are more preferably between 2and 18. Incidentally, E′_(j), E′_(S) and E″_(j), E″_(S) are the averagevalues for the storage modulus or loss modulus measured in the presettemperature range of the measuring apparatus, with 160 to 220° C. set asthe widest temperature range for each. When the separator is in the formof a laminated membrane or in the form of a composite membranecomprising a microporous membrane and an inorganic porous layer, thesilane-modified polyolefin-containing microporous membrane alone isremoved from the laminated membrane or the composite membrane and thestorage modulus E′_(j) and E′_(S) and the loss modulus E″_(j) and E″_(S)of the silane-modified polyolefin-containing microporous membrane aremeasured.

The separator according to the ninth embodiment has:

a mixed storage modulus ratio (R_(E′mix)) of preferably 1.5 to 20 asdefined by the following formula (2A):R _(E′mix) =E′/E′ ₀  (2A){where E′ is the storage modulus measured at 160° C. to 220° C. for theseparator for an electricity storage device and E′₀ is the storagemodulus measured at 160° C. to 220° C. for a separator for anelectricity storage device not containing the silane-modifiedpolyolefin}, and/or

a mixed loss modulus ratio (R_(E″mix)) of preferably 1.5 to 20 asdefined by the following formula (2B):R _(E″mix) =E″/E″ ₀  (2B){where E″ is the loss modulus measured at 160° C. to 220° C. for theseparator for an electricity storage device, and E″₀ is the loss modulusmeasured at 160° C. to 220° C. for a separator for an electricitystorage device not containing the silane-modified polyolefin},when measured after the inorganic porous layer has been removed from theseparator, in the viscoelasticity measurement (version 1) described inthe Examples. Since the mixed storage modulus ratio (R_(E′mix)) and/orthe mixed loss modulus ratio (R_(E″mix)) are within the range of 1.5 to20, it is easy to obtain both a shutdown function and high-temperaturemembrane rupture resistance. The mixed storage modulus ratio (R_(E′mix))and/or mixed loss modulus ratio (R_(E″mix)) are more preferably between2 and 18. Incidentally, E′ and E′₀, and E″ and E″₀, are the averagevalues for the storage modulus or loss modulus measured in the presettemperature range of the measuring apparatus, with 160 to 220° C. set asthe widest temperature range for each. When the separator is in the formof a laminated membrane or in the form of a composite membranecomprising a microporous membrane and an inorganic porous layer, thesilane-modified polyolefin-containing microporous membrane alone isremoved from the laminated membrane or the composite membrane and thestorage modulus E′ and E′₀ and the loss modulus E″ and E″₀ of thesilane-modified polyolefin-containing microporous membrane are measured.A separator for an electricity storage device not containing asilane-modified polyolefin will be described in detail in the Examples.

From the viewpoint of obtaining both a shutdown function andhigh-temperature membrane rupture resistance, the separator according tothe ninth embodiment preferably has a transition temperature of 135° C.to 150° C. for the rubber plateau and the crystal melt flow region, forthe temperature-dependent change of the storage modulus. The transitiontemperature of the rubber plateau and crystal melt flow region ispreferably 137° C. to 147° C., more preferably 140° C. to 145° C. andeven more preferably 140° C. to 143° C. When the separator is in theform of a laminated stack or in the form of a composite membranecomprising a microporous membrane and an inorganic porous layer, thesilane-modified polyolefin-containing microporous membrane alone isremoved from the laminated membrane or the composite membrane andtransition temperature of the silane-modified polyolefin-containingmicroporous membrane are measured.

Tenth Embodiment

The separator for an electricity storage device according to a tenthembodiment of the invention (hereunder referred to simply as“separator”) comprises a first porous layer (layer A) that includes asilane-modified polyolefin and is able to form a crosslinked structure,and a second porous layer (layer B) that includes inorganic particles.Layer A and layer B are both either single layers or multiple layers.Layer B is formed on only one side or on both sides of layer A.

In a LIB, as a typical electricity storage device, lithium (Li) ionsreciprocate between positive and negative electrodes. By situating aseparator comprising layer A and layer B between the positive andnegative electrodes, therefore, it is possible to cause relatively rapidmovement of Li ions between the positive and negative electrodes, whileavoiding contact between the positive and negative electrodes.

(Thickness Ratio)

Layer A functions as a crosslinkable microporous membrane, while layer Bfunctions as an inorganic porous layer to be formed on the microporousmembrane.

The ratio of the thickness (TA) of layer A with respect to the thickness(TB) of layer B (TA/TB) is preferably 0.22 to 14. If the ratio (TA/TB)is 0.22 or greater it will be possible to adequately ensure the presenceof layer A in the separator and to thus exhibit the function of layer A.If the ratio (TA/TB) is 14 or lower, it will be possible to adequatelyensure the presence of layer B in the separator and to thus exhibit thefunction of layer B.

By forming layer A and layer B with their respective specific structuresand setting the ratio (TA/TB) to be within this range, it is possible toprovide a separator that can improve cycle characteristics and safety inan electricity storage device. The separator can be suitably used as aconstituent material of a LIB for mounting in a mobile device or avehicle.

From the viewpoint of this effect, the ratio (TA/TB) is preferably 0.8or greater and more preferably 1.0 or greater. The ratio (TA/TB) is alsopreferably no higher than 5.5 and more preferably no higher than 3.2.

The ratio (TA/TB) may be set to be lower than 2.5, 2.0 or lower, or 1.0or lower, for example. In this case, the thickness (TA) of layer A isless than 2.5 times the thickness (TB) of layer B, or even smaller thanthe thickness (TB) of layer B, thus allowing the layer A to be providedas a thinner membrane so that the separator thickness can be reduced.

The total thickness of layer A and layer B (TA+TB) is preferably 3.0 μmto 22 μm. If the total thickness (TA+TB) is 3.0 μm or greater themembrane strength of the separator will tend to be increased. If thetotal thickness (TA+TB) is 22 μm or smaller, on the other hand, the ionpermeability of the separator will tend to be increased.

From the viewpoint of this effect, the total thickness (TA+TB) is morepreferably 3.5 μm or greater and even more preferably 4.0 μm or greater.The total thickness (TA+TB) is also more preferably no greater than 20μm and even more preferably no greater than 18 μm.

The total thickness (TA+TB) may be set to less than 11 μm, 10 μm orsmaller or 8 μm or smaller, for example. Even with such a thin separatorit is still possible to improve the cycle characteristics and safety ofan electricity storage device, so long as the ranges of the inventionare satisfied.

The ratio (TA/TB) and the total thickness (TA+TB) may each be measuredby the methods described in the Examples, and they can be controlled byadjusting the thickness (TA) and/or the thickness (TB). Layer A andlayer B will now be described.

(Shutdown Temperature and Meltdown Temperature)

Layer A preferably has a shutdown temperature (also referred to as thefuse temperature) of 130° C. to 160° C. and a meltdown temperature (alsoreferred to as the membrane rupture temperature) of 200° C. or higher,as measured based on the electrical resistance under pressure of 0.1 MPato 10.0 MPa (preferably under pressure of 10 MPa).

If the shutdown temperature is 130° C. or higher it will be possible toavoid unnecessary operation of the shutdown function during periods ofnormal reaction in the electricity storage device, and the electricitystorage device can be ensured to have sufficient output characteristics.If the shutdown temperature is 160° C. or lower, on the other hand, theshutdown function can be suitably exhibited during periods of abnormalreaction in the electricity storage device.

In addition, a shutdown temperature of 200° C. or higher will be able tostop abnormal reaction before the ultra-high temperature range isreached, during periods of abnormal reaction in the electricity storagedevice, and can prevent melting membrane rupture of the separator duringperiods of abnormal reaction of the electricity storage device.

In other words, if the shutdown temperature and meltdown temperaturesatisfy the conditions specified above, then it will be possible toobtain a separator that is able to provide an electricity storage devicewith excellent heat resistance, pore occlusion property (shutdownfunction) and melting membrane rupture property (meltdown function), andto ensure the mechanical properties and ion permeability of theseparator itself. With a separator whose shutdown temperature andmeltdown temperature satisfy the aforementioned conditions, therefore,the electricity storage device can be designed with improved cyclecharacteristics and safety.

From the viewpoint of this effect, the shutdown temperature ispreferably higher than 130° C., more preferably 135° C. or higher andeven more preferably 136° C. or higher. The shutdown temperature is alsopreferably no higher than 150° C., more preferably no higher than 148°C. and even more preferably no higher than 146° C.

From the same viewpoint of this effect, the meltdown temperature ispreferably 175° C. or higher, more preferably 178° C. or higher and evenmore preferably 180° C. or higher. The meltdown temperature is alsopreferably no higher than 230° C., more preferably no higher than 225°C. and even more preferably no higher than 220° C.

The condition of “a meltdown temperature of 200° C. or higher” issatisfied even when the meltdown temperature cannot be accuratelymeasured in the range exceeding 200° C., so long as the temperature is200° C. or higher.

The terms “shutdown temperature” and “meltdown temperature” as usedherein are the values obtained upon measurement based on the electricalresistance under the pressure specified above. Specifically, theshutdown temperature and meltdown temperature are derived from thealternating-current resistance (alternating-current resistance betweenelectrodes) that increases with increasing temperature of the laminatedstack comprising the positive electrode, separator and negativeelectrode while applying the aforementioned pressure to the laminatedstack. For the tenth embodiment, the shutdown temperature is thetemperature at which the alternating-current resistance first exceeds aprescribed reference value (for example, 1000Ω), and the meltdowntemperature is the temperature at which the alternating-currentresistance exceeding the reference value falls below the reference value(for example, 1000Ω) as further heating is continued thereafter.

A hydraulic jack may be used for pressurizing of the laminated stack,but this is not limitative, and any known pressurizing means other thana hydraulic jack may be used. An aluminum heater may be used for heatingof the laminated stack, but this is also not restrictive, and any knownheating means other than an aluminum heater may be used.

The shutdown temperature and meltdown temperature may be measured by themethods described in the Examples, and they can be controlled byadjusting the structure of and production method for layer A.

(Heat Shrinkage Factor at 150° C.)

The heat shrinkage factor (T2) at 150° C. after formation of thecrosslinked structure in layer A is 0.02 to 0.91 times the heatshrinkage factor (T1) at 150° C. before formation of the crosslinkedstructure. In other words, the ratio of the heat shrinkage factor (T2)at 150° C. after formation of the crosslinked structure in layer A withrespect to the heat shrinkage factor (T1) at 150° C. before formation ofthe crosslinked structure (T2/T1) is 0.02 to 0.91. The heat shrinkagefactor used here is the larger value of the heat shrinkage factor in themachine direction (MD) of layer A and the heat shrinkage factor in thetransverse direction (TD) of layer A.

It is because layer A is able to form a crosslinked structure with asilane-modified polyolefin, that it is possible to notice a change inheat shrinkage factor before and after crosslinking.

If the ratio (T2/T1) is 0.02 or greater it will be possible toeffectively inhibit short circuiting, thereby reliably preventingtemperature increase of the electricity storage device as a whole andconsequent generation of fumes or ignition. It may be judged thatcrosslinking reaction in layer A has proceeded sufficiently if the ratio(T2/T1) is no greater than 0.91. That is, if the ratio (T2/T1) is withinthe range specified above, a separator for an electricity storage devicecan be provided that improves the cycle characteristics and safety foran electricity storage device.

From the viewpoint of this effect, the ratio (T2/T1) is preferably 0.03or greater, more preferably 0.05 or greater and even more preferably0.07 or greater. The ratio (T2/T1) is also preferably no greater than0.7, more preferably no greater than 0.5 and even more preferably nogreater than 0.4.

The heat shrinkage factor (T1) at 150° C. before formation of thecrosslinked structure is preferably no higher than 70% and morepreferably no higher than 60%.

The heat shrinkage factor (T2) at 150° C. after formation of thecrosslinked structure is preferably no higher than 60% and morepreferably no higher than 50%. However, since formation of a crosslinkedstructure tends to result in a lower heat shrinkage factor compared tobefore formation of the crosslinked structure, the heat shrinkage factor(T2) will generally be a smaller value than the heat shrinkage factor(T1).

The heat shrinkage factor at 150° C. can be measured by the methodsdescribed in the Examples, and they can be controlled by adjusting thestructure of and production method for layer A.

The separators of the embodiments described above are interchangeableand may also be combined with each other. The separator of the ninth ortenth embodiment described above may also optionally include a layerother than the microporous membrane and inorganic porous layer. Theconstituent elements of the separators of the first to tenth embodimentswill now be described.

[Microporous Membrane]

The microporous membrane may be formed of a polyolefin or a modifiedpolyolefin.

The microporous membrane includes a silane-modified polyolefin, and mayoptionally include other polyolefins. Due to the silane crosslinkingproperty of the silane-modified polyolefin, the microporous membrane isable to undergo crosslinking reaction during the production process forthe separator.

The polyolefin to be included in the microporous membrane is notparticularly restricted, and examples include ethylene or propylenehomopolymers, and copolymers formed from two or more monomers selectedfrom the group consisting of ethylene, propylene, 1-butene,4-methyl-1-pentene, 1-hexene, 1-octene and norbornane. Among these,high-density polyethylene (homopolymer) or low-density polyethylene ispreferred, and high-density polyethylene (homopolymer) is morepreferred, from the viewpoint of allowing heat setting (also abbreviatedas “HS”) to be carried out at higher temperature while avoidingobstruction of the pores. A single polyolefin may be used alone, or twoor more may be used in combination.

From the viewpoint of redox degradation resistance and obtaining acompact, homogeneous porous body structure, the microporous membrane ispreferably produced using both a silane-modified polyolefin andultrahigh molecular weight polyethylene (UHMWPE) as starting materials.The weight-average molecular weight of ultrahigh molecular weightpolyethylene (UHMWPE) is generally known to be 1,000,000 or higher. Morespecifically, the weight ratio of the silane-modified polyolefin andUHMWPE during production of the microporous membrane or separator(silane-modified polyolefin weight/UHMWPE weight) is 0.05/0.95 to0.40/0.60.

The content of the polyolefin in the microporous membrane is preferably50 wt % to 100 wt %, preferably 70 wt % to 100 wt % or preferably 80 wt% to 100 wt %. The microporous membrane also preferably includes apolyolefin with a weight-average molecular weight of 100,000 or higherand less than 1,000,000 (included in a proportion of preferably 40 wt %or greater and more preferably 80 wt % or greater with respect to theentire polyolefin). The weight-average molecular weight of thepolyolefin is more preferably 120,000 or higher and less than 950,000,and even more preferably 130,000 or higher and less than 930,000. Byusing a polyolefin having a weight-average molecular weight of 100,000or higher and less than 1,000,000, relaxation of shrinkage of thepolymer will take place early during a heating test of the electricitystorage device, and in particular, safety will be more easily maintainedin the heating safety test. By adjusting the weight-average molecularweight of the microporous membrane to lower than 1,000,000 it ispossible to inhibit casting defects (film patterns) during extrusion,known as “melt fracture”. By adjusting the weight-average molecularweight of the microporous membrane to 100,000 or higher, on the otherhand, it is possible to inhibit transfer of recesses when themicroporous membrane has been wound around a core (winding core).

The viscosity-average molecular weight of the microporous membraneduring removal of the inorganic porous layer and during uncrosslinkedtreatment is preferably 100,000 to 1,200,000 and more preferably 150,000to 800,000, from the viewpoint of avoiding generation of polymer powderby abrasive shear when the separator is transported by a roll.

The membrane thickness of the microporous membrane is preferably 1.0 μmor greater, more preferably 2.0 μm or greater and even more preferably3.0 μm or greater, 4.0 μm or greater or 4.5 μm or greater. A microporousmembrane thickness of 1.0 μm or greater will tend to result in increasedmembrane strength. The membrane thickness of the microporous membrane isalso preferably no greater than 500 μm, more preferably no greater than100 μm and more preferably no greater than 80 μm, no greater than 22 μmor no greater than 19 μm. A microporous membrane thickness of no greaterthan 500 μm will tend to result in increased ion permeability. Themembrane thickness of the microporous membrane can be measured by themethod described in the Examples.

When the microporous membrane is a separator to be used in a relativelyhigh-capacity lithium ion secondary battery of recent years, themembrane thickness of the microporous membrane is preferably no greaterthan 25 μm, more preferably no greater than 22 μm or no greater than 20μm, even more preferably no greater than 18 μm and most preferably nogreater than 16 μm. In this case, a microporous membrane thickness of nogreater than 25 μm will tend to result in increased permeability. Thelower limit for the microporous membrane thickness may be 1.0 μm orgreater, 3.0 μm or greater, 4.0 μm or greater, 6.0 μm or greater or 7.5μm or greater.

From the viewpoint of the high-temperature membrane rupture resistanceof the separator for an electricity storage device and the safety of theelectricity storage device, a microporous membrane used as the separatorhas a melted membrane rupture temperature of preferably 180° C. to 220°C. and more preferably 180° C. to 200° C., as measured bythermomechanical analysis (TMA). In most cases when an electricitystorage device has released heat in an unexpected runaway reaction, thepolyolefin separator for an electricity storage device fuses at lowtemperature (for example, 150° C. or below), resulting in earlymigration of Li ions and consequent halting of discharge inside oroutside of the electricity storage device. By subsequent cooling of theelectricity storage device by external air or a coolant, the electricitystorage device as a whole is cooled and it is possible to inhibitignition of the electrolyte solution or exothermic decompositionreaction of the electrolyte, thus allowing the safety to be ensured.However, runaway reaction occurring inside the electricity storagedevice is not stopped by fusing of the separator, and heat releasecontinues until the separator undergoes molten membrane rupture, makingit impossible to ensure the safety of the device. It is thereforeimportant for the separator to not undergo molten membrane rupture untilthe electricity storage device as a whole has been thoroughly cooled.Moreover, in extreme cases where the temperature has increased to theultra-high temperature range of 220° C. or higher, decompositionreaction of the electrolyte solution or electrolyte proceeds violently,with the decomposition products causing corrosion reaction on theelectrodes, or heat also being released to the point of explosion. Insuch cases, the separator undergoes molten membrane rupture, seepinginto both electrodes and coating the active materials, thus allowingcorrosion reaction to be inhibited.

[First Porous Layer (Layer A)]

Layer A comprises a silane-modified polyolefin and can form acrosslinked structure. From the viewpoint of ensuring degradationresistance against oxidation-reduction and ensuring a compact,homogeneous porous body structure, layer A preferably further includespolyethylene as a different polyolefin from the silane-modifiedpolyolefin. Layer A may also include components other than thesilane-modified polyolefin and polyethylene.

The polyolefin composing the silane-modified polyolefin in layer A maybe a homopolymer of ethylene or propylene; or a copolymer formed from atleast two monomers selected from the group consisting of ethylene,propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene andnorbornane. Among these, the polyolefin is preferably ethylenehomopolymer (polyethylene), more preferably high-density polyethyleneand/or low-density polyethylene and even more preferably high-densitypolyethylene, from the viewpoint of allowing heat setting at highertemperature while avoiding obstruction of the pores. A single polyolefinmay be used alone, or two or more may be used in combination.

Layer A may also include a polymer (another polymer) other than asilane-modified polyolefin or polyethylene, within a range that does notoverly inhibit the effect of the invention.

The weight-average molecular weight of layer A as a whole is preferably100,000 to 1,200,000 and more preferably 150,000 to 800,000.

(Thickness of Layer A)

The thickness (TA) of layer A is preferably 1 μm or greater, morepreferably 2 μm or greater and even more preferably 3 μm or greater. Ifthe thickness (TA) is 1 μm or greater the membrane strength will tend tobe further increased. The thickness (TA) of layer A is also preferably500 μm or smaller, more preferably 100 μm or smaller and even morepreferably 80 μm or smaller. If the thickness (TA) is 500 μm or smallerthe ion permeability will tend to be further increased. The thickness(TA) may be set to 1.00 μm or greater, 2.00 μm or greater or 3.00 μm orgreater, for example.

When the separator is a separator for a LIB, the thickness (TA) ispreferably less than 22 μm, more preferably no greater than 21 μm andeven more preferably no greater than 20.5 μm. When the separator is aseparator for a LIB the upper limit for the thickness (TA) may be set toless than 13 μm or no greater than 8.5 μm. If the thickness (TA) is 25μm or smaller the ion permeability will tend to be further increased.The thickness (TA) may be set to less than 22.00 μm, 21.00 μm orsmaller, 20.00 μm or smaller, less than 13.00 μm or 8.50 or smaller. Thelower limit for the thickness (TA) may be the same as described above.

The thickness (TA) can be measured by the method described in theExamples, and it can be controlled by varying the stretch ratio of layerA.

When layer A is a single layer, the thickness of layer A is treated asthe thickness (TA). When layer A consists of multiple layers, the totalof the thicknesses of the multiple layers in layer A is treated as thethickness (TA).

(Membrane Rupture Temperature of Layer A)

The membrane rupture temperature of layer A is preferably 180° C. to220° C., as measured by thermomechanical analysis (TMA).

Even when the electricity storage device has generated abnormal heatrelease due to runaway reaction, the shutdown function of the separatoris expected to stop movement of Li ions, and discharge in theelectricity storage device or outside of the electricity storage devicethat results from it. It is expected that the electricity storage deviceas a whole will then be cooled by a coolant, thus ensuring the safety.On the other hand, if the membrane rupture temperature is within therange specified above, then the separator will undergo molten ruptureand seep onto both electrodes so that the active materials can becoated, thus even more easily inhibiting heat release even when theelectricity storage device as a whole is not sufficiently cooled, oreven if an ultra-high temperature range is reached.

The membrane rupture temperature can be measured by the method describedin the Examples, and it can be controlled by changing the stretchingtemperature and/or stretch ratio during the production process.

(Porosity of Microporous Membrane or Layer A)

The porosity of the microporous membrane or layer A is preferably 20% orgreater, more preferably 25% or greater, and even more preferably 28% orgreater, 30% or greater, 32% or greater or 35% or greater. If theporosity is 20% or greater, its ability to follow rapid movement of Liions will be further increased. The porosity is also preferably nogreater than 90%, more preferably no greater than 80% and even morepreferably no greater than 60%. If the porosity is no greater than 90%,the membrane strength will be further increased and self-discharge willtend to be inhibited.

The porosity can be measured by the method described in the Examples,and it can be controlled by changing the stretching temperature and/orstretch ratio during the production process.

(Air Permeability of Microporous Membrane or Layer A)

The air permeability of the microporous membrane or layer A ispreferably 1 second/100 cm³ or greater, more preferably 50 seconds/100cm³ or greater, even more preferably 55 seconds/100 cm³ or greater, andyet more preferably 70 seconds or greater, 90 seconds or greater or 110seconds or greater. If the air permeability is 1 second/100 cm³ orgreater, the balance between the membrane thickness, porosity and meanpore size will tend to be improved. The air permeability is alsopreferably no greater than 400 seconds/100 cm³, more preferably nogreater than 300 seconds/100 cm³ and even more preferably no greaterthan 270 seconds/100 cm³. If the air permeability is no greater than 400seconds/100 cm³, the ion permeability will tend to be further increased.

The air permeability can be measured by the method described in theExamples, and it can be controlled by changing the stretchingtemperature and/or stretch ratio during the production process.

(Puncture Strength of Microporous Membrane or Layer A)

The puncture strength of the microporous membrane or layer A ispreferably 200 gf/20 μm or greater and more preferably 300 gf/20 μm orgreater. If the puncture strength is 200 gf/20 μm or greater, then evenif active materials have dropped out when the laminated stack of theseparator and electrodes has been wound, it will be easier to inhibitmembrane rupture due to the dropped out active materials. It will alsobe possible to reduce the possibility of short circuiting caused byexpansion and contraction of the electrodes during charge-discharge. Thepuncture strength of the microporous membrane or layer A is alsopreferably no greater than 4000 gf/20 μm and more preferably no greaterthan 3800 gf/20 μm. If the puncture strength is no greater than 3500gf/20 μm, then it will be easier to reduce heat shrinkage duringheating.

The puncture strength can be measured by the method described in theExamples, and it can be controlled by changing the stretchingtemperature and/or stretch ratio during the production process.

[Tensile Strength of Microporous Membrane or Layer A]

The tensile strength of the microporous membrane or layer A ispreferably 1000 kgf/cm² or greater, more preferably 1050 kgf/cm² orgreater and even more preferably 1100 kgf/cm² or greater in both the MD(the lengthwise direction, machine direction or flow direction of themembrane or layer A) and the TD (the direction perpendicular to the MD,i.e. the transverse direction of the membrane or layer A). If thetensile strength is 1000 kgf/cm² or greater, then slitting or ruptureduring winding of the electricity storage device will tend to be furtherinhibited, or short circuiting due to contaminants in the electricitystorage device will tend to be further inhibited. The tensile strengthis also preferably no greater than 5000 kgf/cm², more preferably nogreater than 4500 kgf/cm² and even more preferably no greater than 4000kgf/cm². If the tensile strength is no greater than 5000 kgf/cm², thenthe microporous membrane or layer A will undergo earlier relaxation toexhibit weaker contractive force during heat testing, thus tending toresult in higher safety.

[Tensile Modulus of Microporous Membrane or Layer A]

The tensile modulus of the microporous membrane or layer A is preferablyno greater than 120 N/cm, more preferably no greater than 100 N/cm andmore preferably no greater than 90 N/cm, in both the MD and TD. Atensile modulus of no greater than 120 N/cm means that the separator fora lithium ion secondary battery is not excessively oriented, and forexample, when the plugging agent such as polyethylene melts and shrinksin a heating test, it will tend to allow the polyethylene to undergoearly stress relaxation, thereby preventing shrinkage of the separatorin the battery and being more likely to prevent short circuiting betweenthe electrodes (that is, it can improve the safety of the separatorduring heating). A low tensile modulus in this range is easily achievedby including polyethylene with a weight-average molecular weight of500,000 or lower in the polyolefin forming the microporous membrane orlayer A. The lower limit for the tensile modulus, on the other hand, isnot particularly restricted but is preferably 10 N/cm or greater, morepreferably 30 N/cm or greater and even more preferably 50 N/cm orgreater. The tensile modulus can be appropriately adjusted by adjustingthe degree of stretching in the production process or by relaxation asnecessary after stretching.

<Polyolefin>

The polyolefin is not particularly restricted, and examples includeethylene or propylene homopolymers, and copolymers formed from two ormore monomers selected from the group consisting of ethylene, propylene,1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and norbornane. Amongthese, high-density polyethylene or low-density polyethylene ispreferred, and high-density polyethylene is more preferred, from theviewpoint of allowing heat setting (also abbreviated as “HS”) to becarried out at higher temperature while avoiding obstruction of thepores. A single polyolefin may be used alone, or two or more may be usedin combination.

The separator also preferably includes a polyolefin with aweight-average molecular weight (Mw) of less than 2,000,000, with thepolyolefin with a Mw of less than 2,000,000 included in a proportion ofmore preferably 40 weight % or greater and even more preferably 80weight % or greater with respect to the entire polyolefin. By using apolyolefin with an Mw of less than 2,000,000, relaxation of shrinkage ofthe polymer will take place early during a heating test of theelectricity storage device, and in particular, safety will be moreeasily maintained in a heating safety test. When a polyolefin with an Mwof less than 2,000,000 is used, the elastic modulus in the thicknessdirection of the obtained microporous membrane tends to be lowercompared to when a polyolefin of 1,000,000 or higher is used, andtherefore a microporous membrane is obtained with relatively easiertransfer of core irregularities. The weight-average molecular weight ofthe entire polyolefin microporous membrane composing the separator ispreferably 100,000 to 2,000,000 and more preferably 150,000 to1,500,000.

(Polyolefin with One or More Types of Functional Groups)

From the viewpoint of formation of a crosslinked structure, redoxdegradation resistance and obtaining a compact, homogeneous porous bodystructure, the separator preferably comprises a functionalgroup-modified polyolefin or a polyolefin in which monomers withfunctional groups are copolymerized, as the polyolefin with one or moretypes of functional groups. As used herein, a “functional group-modifiedpolyolefin” is a compound in which the functional groups are bondedafter production of the polyolefin. The functional groups may be bondedto the polyolefin backbone or they may be ones that can be introducedinto a comonomer, and preferably they contribute to selectivecrosslinking of the amorphous portion of the polyolefin, with examplesincluding one or more selected from the group consisting of carboxyl,hydroxyl, carbonyl, polymerizable unsaturated hydrocarbon, isocyanate,epoxy, silanol, hydrazide, carbodiimide, oxazoline, acetoacetyl,aziridine, ester, active ester, carbonate, azide, straight-chain orcyclic heteroatom-containing hydrocarbon, amino, sulfhydryl, metalchelating and halogen-containing groups.

From the viewpoint of the separator strength, ion permeability, redoxdegradation resistance and compact and homogeneous porous bodystructure, the separator preferably comprises both a polyolefin with oneor more types of functional groups and a silane-unmodified polyethylene.When a polyolefin with one or more types of functional groups and asilane-unmodified polyethylene are combined, preferably the weight ratioof the polyolefin with one or more types of functional groups and thesilane-unmodified polyethylene in the separator (weight of polyolefinwith one or more types of functional groups/weight of silane-unmodifiedpolyethylene) is 0.05/0.95 to 0.80/0.20.

(Crosslinked Structure)

The crosslinked structure of the separator contributes to achieving botha shutdown function and high-temperature membrane rupture resistance forthe separator and to safety of the electricity storage device, andpreferably it is formed in the amorphous portion of the polyolefin ofthe separator. The crosslinked structure can be formed by reaction viacovalent bonding, hydrogen bonding or coordination bonding, for example.The reaction by covalent bonding is preferably one or more selected fromthe group consisting of the following reactions (I) to (IV):

(I) condensation reaction of a plurality of the same functional groups

(II) reaction between a plurality of different functional groups

(III) chain condensation reaction between a functional group and theelectrolyte solution

(IV) chain condensation reaction between a functional group and anadditive.

The reaction by coordination bonding is preferably the followingreaction (V):

(V) reaction in which a plurality of the same functional groupscrosslink by coordination bonding with eluting metal ions.

Reaction (I)

A schematic scheme and specific example of reaction (I) is shown below,with the first functional group of the separator represented as A.

{In the formula, R is an optionally a substituted alkyl group or heteroalkyl group of 1 to 20 carbon atoms.}

When functional group A for reaction (I) is a silanol group, thepolyolefin in the separator is preferably silane graft-modified. Asilane graft-modified polyolefin is composed with a structure having apolyolefin as the main chain and alkoxysilyl groups grafted onto themain chain. The alkoxide substituted on the alkoxysilyl group may bemethoxide, ethoxide or butoxide, for example. For example, R in theformula may be methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl,isobutyl or tert-butyl. The main chain and grafts may be linked bycovalent bonding, for an alkyl, ether, glycol or ester structure. Inconsideration of the production process for the separator of thisembodiment, the silane graft-modified polyolefin preferably has asilicon-to-carbon ratio (Si/C) of 0.2 to 1.8% and more preferably 0.5 to1.7%, at the stage before the crosslinking step.

A preferred silane graft-modified polyolefin is one with a density of0.90 to 0.96 g/cm³ and a melt mass-flow rate (MFR) of 0.2 to 5 g/min at190° C. From the viewpoint of inhibiting generation of resin aggregatesduring the production process for the separator, and maintaining silanecrosslinkability until contact with the electrolyte solution, the silanegraft-modified polyolefin is preferably not a master batch resincontaining a dehydrating condensation catalyst. Dehydrating condensationcatalysts are also known to function as catalysts for siloxanebond-forming reactions with alkoxysilyl group-containing resins. Theterm “master batch resin” will be used herein to refer to a compoundedproduct obtained by pre-adding a dehydrating condensation catalyst (forexample, an organometallic catalyst) to an alkoxysilyl group-containingresin or other kneading resin in a continuous process of kneading aresin using an extruder.

Reaction (II)

A schematic scheme and specific example of reaction (II) is shown below,with the first functional group of the separator represented as A andthe second functional group represented as B.

Reaction (I) and reaction (II) are subject to catalytic action, and forexample, they can be catalytically accelerated by a chemical substanceinside the electricity storage device in which the separator isincorporated. The chemical substance may be, for example, anelectrolyte, electrolyte solution, electrode active material or additivecontained in the electricity storage device, or a decomposition productthereof.

Reaction (III)

A schematic scheme and specific example of reaction (III) is shownbelow, with the first functional group of the separator represented asA, and the electrolyte solution represented as Sol.

Reaction (IV)

A schematic scheme for reaction (IV) is shown below, with the firstfunctional group of the separator represented as A, the optionallyincorporated second functional group as B and an additive as Add.

From the viewpoint of forming the covalent bonds represented by thedotted lines in the scheme, reaction (IV) is preferably nucleophilicsubstitution reaction, nucleophilic addition reaction or ring-openingreaction between the compound Rx composing the separator and thecompound Ry composing the additive (Add). Compound Rx may be thepolyolefin in the separator, such as polyethylene or polypropylene, andpreferably the polyolefin is modified with a functional group x, i.e.modified with one or more selected from the group consisting of —OH,—NH₂, —NH—, —COOH and —SH, for example.

Since multiple compounds Rx are crosslinked by compound Ry as theadditive, compound Ry preferably has two or more linking reaction units(y₁). The multiple linking reaction units y₁ may be any groups with anystructure so long as they are able to participate in nucleophilicsubstitution reaction, nucleophilic addition reaction or ring-openingreaction with the functional group x of compound Rx, and they may besubstituted or unsubstituted, may contain heteroatoms or inorganicmaterials, and may be the same or different from each other. Whencompound Ry has a straight-chain structure, the multiple linkingreaction units (y₁) may each independently be end groups or groupsincorporated into the main chain, or side chain or pendant groups.

When reaction (IV) is nucleophilic substitution reaction it may be asfollows, considering the functional group x of compound Rx to be thenucleophilic group and the linking reaction unit y₁ of compound Ry to bethe leaving group, but this is only an example, and for the purpose ofthis embodiment the functional group x and linking reaction unit y₁ mayboth be leaving groups, depending on their nucleophilicity.

From the viewpoint of the nucleophilic reagent, the functional group xof compound Rx is preferably an oxygen-based nucleophilic group,nitrogen-based nucleophilic group or sulfur-based nucleophilic group.Oxygen-based nucleophilic groups include hydroxyl, alkoxy, ether andcarboxyl groups, of which —OH and —COOH are preferred. Nitrogen-basednucleophilic groups include ammonium groups, primary amino groups andsecondary amino groups, of which —NH₂ and —NH— are preferred.Sulfur-based nucleophilic groups include —SH and thioether groups, forexample, with —SH being preferred.

When reaction (IV) is nucleophilic substitution reaction, from theviewpoint of the leaving group, the linking reaction unit y₁ of compoundRy is preferably an alkylsulfonyl group such as CH₃SO₂— or CH₃CH₂SO₂—;an arylsulfonyl group (—ArSO₂—); a haloalkylsulfonyl group such asCF₃SO₂— or CCl₃SO₂—; an alkyl sulfonate group such as CH₃SO₃ ⁻— orCH₃CH₂SO₃ ⁻—; an aryl sulfonate group (ArSO₃ ⁻—); a haloalkyl sulfonategroup such as CF₃SO₃ ⁻— or CCl₃SO₃ ⁻—; or a heterocyclic group, any ofwhich may be used alone or in combinations of two or more differentones. Heteroatoms in a heterocyclic ring include nitrogen atoms, oxygenatoms and sulfur atoms, with nitrogen atoms being preferred from theviewpoint of dissociability. The leaving group containing a nitrogenatom in the heterocyclic ring is preferably a monovalent grouprepresented by one of the following formulas (y₁-1) to (y₁-6).

{where X is hydrogen or a monovalent substituent.}

{where X is hydrogen or a monovalent substituent.}

{where X is hydrogen or a monovalent substituent.}

{where X is hydrogen or a monovalent substituent.}

{where X is hydrogen or a monovalent substituent.}

{where X is hydrogen or a monovalent substituent.}

In formulas (y1-1) to (y₁-6), X is hydrogen or a monovalent substituent.Examples of monovalent substituents include alkyl groups, haloalkylgroups, alkoxyl groups and halogen atoms.

When reaction (IV) is nucleophilic substitution reaction and compound Ryhas a straight-chain structure, compound Ry preferably has, as astraight-chain unit y₂ in addition to the linking reaction unit y₁, oneor more selected from the group consisting of divalent groupsrepresented by the following formulas (y₂-1) to (y₂-6).

{where m is an integer of 0 to 20, and n is an integer of 1 to 20.}

{where n is an integer of 1 to 20.}

{where n is an integer of 1 to 20.}

{where n is an integer of 1 to 20.}

{where X is an alkylene or arylene group of 1 to 20 carbon atoms, and nis an integer of 1 to 20.}

{where X is an alkylene or arylene group of 1 to 20 carbon atoms, and nis an integer of 1 to 20}. When compound Ry contains multiplestraight-chain units y₂, they may be the same or different, and theirsequences may be either block or random.

In formula (y₂-1), m is an integer of 0 to 20, and from the viewpoint ofthe crosslinked network it is preferably 1 to 18. In formulas (y₂-1) to(y₂-6), n is an integer of 1 to 20, and from the viewpoint of thecrosslinked network it is preferably 2 to 19 or 3 to 16. In formula(y₂-5) to (y₂-6), X is an alkylene or arylene group of 1 to 20 carbonatoms, and from the viewpoint of stability of the straight-chainstructure it is preferably a methylene, ethylene, n-propylene,n-butylene, n-hexylene, n-heptylene, n-octylene, n-dodecylene,o-phenylene, m-phenylene or p-phenylene group.

Tables 2 to 4 below show preferred combinations for the functional groupx of compound Rx and the linking reaction unit y₁ and straight-chainunit y₂ of compound Ry, when reaction (IV) is nucleophilic substitutionreaction.

TABLE 2 Nucleophilic substitution reaction (preferred combination I)Separator functional group Additive (compound Ry) (functional group x ofTwo or more linking reaction units (y1) compound Rx) Straight-chain unit(y2) Both terminals —OH —NH₂ —NH— —COOH —SH

TABLE 3 Nucleophilic substitution reaction (preferred combination II)Separator functional group Additive (compound Ry) (functional group x ofTwo or more linking reaction units (y1) compound Rx) Straight-chain unit(y2) Both terminals —OH —NH₂ —NH— —COOH —SH

CF₃SO₂— CH₃SO₂— ArSO₂— CF₃SO₃ ⁻— CH₃SO₃ ⁻— ArSO₃ ⁻—

TABLE 4 Nucleophilic substitution reaction (preferred combination III)Separator functional group Additive (compound Ry) (functional group x ofTwo or more linking reaction units (y1) compound Rx) Straight-chain unit(y2) Terminal 1 Terminal 2 —OH —NH₂ —NH— —COOH —SH

The following is a reaction scheme as Specific Example 1 of thenucleophilic substitution reaction, where the functional group x of thepolyolefin is —NH₂, the linking reaction unit y₁ of the additive(compound Ry) is the backbone of a succinimide, and the straight-chainunit (y₂) is —(O—C₂H₅)_(n).

The following is a reaction scheme as Specific Example 2 of thenucleophilic substitution reaction, where the functional groups x of thepolyolefin are —SH and —NH₂, the linking reaction unit y₁ of theadditive (compound Ry) is a nitrogen-containing cyclic backbone, and thestraight-chain unit y₂ is o-phenylene.

When reaction (IV) is nucleophilic addition reaction, the functionalgroup x of compound Rx and the linking reaction unit y₁ of compound Rymay participate in addition reaction. For nucleophilic additionreaction, the functional group x of compound Rx is preferably anoxygen-based nucleophilic group, nitrogen-based nucleophilic group orsulfur-based nucleophilic group. Oxygen-based nucleophilic groupsinclude hydroxyl, alkoxy, ether and carboxyl groups, of which —OH and—COOH are preferred. Nitrogen-based nucleophilic groups include ammoniumgroups, primary amino groups and secondary amino groups, of which —NH₂and —NH— are preferred. Sulfur-based nucleophilic groups include —SH andthioether groups, for example, with —SH being preferred.

In nucleophilic addition reaction, from the viewpoint of additionreactivity and ready availability of starting materials, the linkingreaction unit y₁ of compound Ry is preferably one or more selected fromthe group consisting of groups represented by the following formulas(Ay₁-1) to (Ay₁-6).

{where R is hydrogen or a monovalent organic group.}

In formula (Ay₁-4), R is a hydrogen atom or a monovalent organic group,preferably a hydrogen atom or a C₁₋₂₀ alkyl, alicyclic or aromaticgroup, and more preferably a hydrogen atom or a methyl, ethyl,cyclohexyl or phenyl group.

Tables 5 and 6 below show preferred combinations for the functionalgroup x of compound Rx and the linking reaction unit y₁ of compound Ry,when reaction (IV) is nucleophilic addition reaction.

TABLE 5 Nucleophilic addition reaction (preferred combination I)Separator functional group Additive (compound Ry) (functional group x ofcompound Rx) Two or more linking reaction units (y1) —OH —NH₂ —NH— —COOH—SH

TABLE 6 Nucleophilic addition reaction (preferred combination II)Separator functional group Additive (compound Ry) (functional group x ofTwo or more linking compound Rx) reaction units (y1) —OH —NH₂ —NH— —COOH—SH

The following is a reaction scheme as a specific example of thenucleophilic addition reaction, where the functional group x of theseparator is —OH and the linking reaction unit y₁ of the additive(compound Ry) is —NCO.

When reaction (IV) is ring-opening reaction, the functional group x ofcompound Rx and the linking reaction unit y₁ of compound Ry mayparticipate in ring-opening reaction, and from the viewpoint of readyavailability of starting materials it is preferred to open the cyclicstructure on the linking reaction unit y₁ side. From the same viewpoint,the linking reaction unit y₁ is more preferably an epoxy group, evenmore preferably compound Ry has two or more epoxy groups, and yet morepreferably it is a diepoxy compound.

When reaction (IV) is ring-opening reaction, the functional group x ofcompound Rx is preferably one or more selected from the group consistingof —OH, —NH₂, —NH—, —COOH and —SH, and/or the linking reaction unit y₁of compound Ry is preferably two or more groups represented by thefollowing formula (ROy₁-1).

{where the multiple X groups are each independently a hydrogen atom or amonovalent substituent}. In formula (ROy₁-1), the multiple X groups areeach independently a hydrogen atom or a monovalent substituent,preferably a hydrogen atom or a C₁₋₂₀ alkyl, alicyclic or aromaticgroup, and more preferably a hydrogen atom or a methyl, ethyl,cyclohexyl or phenyl group. Table 7 below shows preferred combinationsfor the functional group x of compound Rx and the linking reaction unity₁ of compound Ry, for epoxy ring-opening reaction.

TABLE 7 Epoxy ring-opening reaction (preferred combination) Additive(compound Ry) Two or more linking reaction units (y1) —OH —NH₂ —NH——COOH —SH

Reaction (V)

A schematic scheme for reaction (V) and an example of functional group Aare shown below, with the first functional group of the separatorrepresented as A and the metal ion represented as M^(n+).

Examples for Functional Group A:

—CHO, —COOH, acid anhydride, —COO—, etc.

In this scheme, the metal ion M^(n+) is preferably one eluted from theelectricity storage device (hereunder also referred to as “eluting metalion”), and it may be one or more selected from the group consisting ofZn²⁺, Mn²⁺, Co³⁺, Ni²⁺ and Li⁺, for example. The following is an exampleof coordination bonding when functional group A is —COO⁻.

A specific scheme for reaction (V) is shown below, where functionalgroup A is —COOH and the eluting metal ion is Zn²⁺.

In this scheme, the hydrofluoric acid (HF) may be derived from theelectrolytes, electrolyte solution, electrode active materials andadditives, or their decomposition products or water-absorbed products,that are present in the electricity storage device, depending on thecharge-discharge cycle of the electricity storage device.

<Silane-Modified Polyolefin>

The silane-modified polyolefin has a structure with a polyolefin as themain chain and alkoxysilyl groups grafted onto the main chain. Thesilane-modified polyolefin can be obtained by grafting alkoxysilylgroups onto the main chain of a non-silane-modified polyolefin.

It is presumed that the alkoxysilyl groups are converted to silanolgroups by water hydrolysis, and undergo crosslinking reaction to formsiloxane bonds (with any proportion among structure T1, structure T2 andstructure T3 in the following formula). Alkoxides substituting on thealkoxysilyl groups may be methoxide, ethoxide or butoxide. In thefollowing formula, R may be methyl, ethyl, n-propyl, isopropyl, n-butyl,sec-butyl, isobutyl or tert-butyl.

The main chain and grafts are linked by covalent bonding. The structureof the covalent bonding may be an alkyl, ether, glycol or esterstructure. At the stage before the crosslinking reaction, thesilane-modified polyolefin has a modification degree of no greater than2% of silanol units with respect to the main chain ethylene units.

A preferred silane graft-modified polyolefin is one with a density of0.90 to 0.96 g/cm³ and a melt mass-flow rate (MFR) of 0.2 to 5 g/min at190° C.

From the viewpoint of satisfactorily exhibiting the effect of theinvention, the amount of silane-modified polyolefin is preferably 0.5weight % or greater or 3 weight % or greater, more preferably 4 weight %or greater, and even more preferably 5 weight % or greater or 6 weight %or greater, based on the total weight of the microporous membrane orlayer A. From the viewpoint of cycle properties and safety of theelectricity storage device, the amount of silane-modified polyolefin ispreferably no greater than 40 weight % and more preferably no greaterthan 38 weight %, based on the total weight of the microporous membrane.The amount of silane-modified polyolefin may be 30 weight % or greateror 50 weight % or greater, or even 100 weight %, based on the totalweight of layer A.

The crosslinked structure of the microporous membrane or layer A ispreferably formed by compounds generated inside the electricity storagedevice.

That is, the crosslinked structure of the microporous membrane or layerA is preferably a crosslinked structure with oligosiloxane bonds formedby utilizing swelling of the microporous membrane or layer A and/orcompounds generated inside the electricity storage device when theseparator is contacted with the nonaqueous electrolyte solution duringthe production process for the electricity storage device. Thecrosslinked structure in this case is a crosslinked structure obtainednot by active promotion of the crosslinking reaction during theproduction process for the separator, but rather by active promotion ofthe crosslinking reaction during the production process for theelectricity storage device, and therefore the self-crosslinking propertyof the separator can be maintained until it is housed in the electricitystorage device.

From the viewpoint of inhibiting generation of resin aggregates duringthe production process for the separator, and maintaining silanecrosslinkability until contact with the electrolyte solution, thesilane-modified polyolefin is preferably not a master batch resincontaining a dehydrating condensation catalyst. Dehydrating condensationcatalysts are also known to function as catalysts for siloxanebond-forming reactions with alkoxysilyl group-containing resins.Throughout the present specification, the term “master batch resin” willbe used to refer to a compounded product obtained by pre-adding adehydrating condensation catalyst (for example, an organometalliccatalyst) to an alkoxysilyl group-containing resin or other kneadingresin in a continuous process with a step of kneading a resin using anextruder.

(Polyethylene)

Throughout the present specification, the polyethylene that can befurther included in addition to the silane-modified polyolefin (thepolyethylene further included in the microporous membrane or layer A asa polyolefin different from the silane-modified polyolefin) ispolyethylene that is a homoethylene polymer or an alkane unit-containingcopolymer with a weight-average molecular weight of 100,000 to10,000,000.

When the microporous membrane or layer A further includes polyethyleneas a polyolefin different from the silane-modified polyolefin, itscontent is preferably 20 weight % or greater, more preferably 40 weight% or greater and even more preferably 50 weight % or greater, based onthe total amount of the silane-modified polyolefin and polyethylene. Ifthe polyethylene content is 20 weight % or greater it will tend to beeasier to ensure degradation resistance against oxidation-reduction, anda compact, homogeneous porous body structure can be ensured.

The polyethylene content is also preferably no greater than 97 weight %,more preferably no greater than 96 weight % and even more preferably nogreater than 95 weight %. If the polyethylene content is no greater than97 weight % it will be possible to ensure the content of thesilane-modified polyolefin in the microporous membrane or layer A.

(Detection Method for Silane-Modified Polyolefin in Separator)

When the silane-modified polyolefin in the separator is in a crosslinkedstate it is insoluble or has insufficient solubility in organicsolvents, and it is therefore difficult to directly measure thesilane-modified polyolefin content from the separator. In such cases, aspretreatment for the sample, methyl orthoformate which does not undergosecondary reactions may be used to decompose the siloxane bonds tomethoxysilanol, and then solution NMR measurement may be carried out todetect the silane-modified polyolefin in the separator. The pretreatmentexperiment may be carried out with reference to Japanese PatentPublication No. 3529854 and Japanese Patent Publication No. 3529858.

Specifically, ¹H or ¹³C NMR identification of the silane-modifiedpolyolefin as the starting material used for production of the separatormay be employed in the detection method for the silane-modifiedpolyolefin in the separator. The following are examples of ¹H- and¹³C-NMR measurement methods.

(¹H-Nmr Measurement)

The sample is dissolved in o-dichlorobenzene-d4 at 140° C. and a 1H-NMRspectrum is obtained at a proton resonance frequency of 600 MHz. The¹H-NMR measuring conditions are as follows.

Apparatus: AVANCE NEO 600 by Bruker

Sample tube diameter: 5 lump

Solvent: o-Dichlorobenzene-d4

Measuring temperature: 130° C.

Pulse angle: 30°

Pulse delay time: 1 sec

Number of scans: ≥1000

Sample concentration: 1 wt/vol %

(¹³C-Nmr Measurement)

The sample is dissolved in o-dichlorobenzene-d4 at 140° C. and a ¹³C-NMRspectrum is obtained. The ¹³C-NMR measuring conditions are as follows.

Apparatus: AVANCE NEO 600 by Bruker

Sample tube diameter: 5 mmφ

Solvent: o-Dichlorobenzene-d4

Measuring temperature: 130° C.

Pulse angle: 30°

Pulse delay time: 5 sec

Number of scans: ≥10,000

Sample concentration: 10 wt/vol %

FIGS. 11 and 12 are ¹H and ¹³C-NMR charts for silane-modified polyolefinstarting materials 1 and 2 using two types of polyolefins, wherestarting materials 1 and 2 each have a different melt index (MI), C₃graft amount, C₄ graft amount and/or silanol-modified amount.

The ¹H- and ¹³C-NMR measuring conditions for FIG. 11 are as follows.

(¹H-NMR Measuring Conditions)

Apparatus: Bruker Avance NEO 600

Observation nucleus: ¹H

Observation frequency: 600 MHz

Pulse program: zg30

Pulse delay time: 1 sec

Number of scans: 1024

Measuring temperature: 130° C.

Chemical shift reference: 7.219 ppm (o-DCBz)

Solvent: o-Dichlorobenzene-d4

Sample concentration: 1 wt/vol %

Sample tube: 5 mmφ

(¹³C-NMR Measuring Conditions)

Apparatus: Bruker Avance NEO 600

Observation nucleus: ¹³C

Observation frequency: 150.91 MHz

Pulse program: zgpg30

Pulse delay time: 5 sec

Number of scans: 24,000 or 12,800

Measuring temperature: 130° C.

Chemical shift reference: 132.39 ppm (o-DCBz)

Solvent: o-Dichlorobenzene-d4

Sample concentration: 10 wt/vol %

Sample tube: 5 lump

The ¹H and ¹³C-NMR measuring conditions for FIG. 12 are as follows.

(¹H-NMR Measuring Conditions)

Apparatus: Bruker Avance NEO 600

Observation nucleus: ¹H

Observation frequency: 600 MHz

Pulse program: zg30

Pulse delay time: 1 sec

Number of scans: 1024

Measuring temperature: 130° C.

Chemical shift reference: 7.219 ppm (o-DCBz)

Solvent: o-Dichlorobenzene-d4

Sample concentration: 1 wt/vol %

Sample tube: 5 mmφ

(¹³C-NMR Measuring Conditions)

Apparatus: Bruker Avance NEO 600

Observation nucleus: ¹³C

Observation frequency: 150.91 MHz

Pulse program: zgpg30

Pulse delay time: 5 sec

Number of scans: 12,800

Measuring temperature: 130° C.

Chemical shift reference: 132.39 ppm (o-DCBz)

Solvent: o-Dichlorobenzene-d4

Sample concentration: 10 wt/vol %

Sample tube: 5 lump

FIG. 13 is a ¹H- and ¹³C-NMR chart of the separator fabricated usingsilane-modified polyolefin starting material 1 shown in FIG. 11 , in thestate before crosslinking, for Example I-1 described below. The ¹H- and¹³C-NMR measuring conditions for FIG. 13 are as follows.

(¹H-NMR Measuring Conditions)

Apparatus: Bruker Avance NEO 600

Observation nucleus: ¹H

Observation frequency: 600 MHz

Pulse program: zg30

Pulse delay time: 1 sec

Number of scans: 1024

Measuring temperature: 130° C.

Chemical shift reference: 7.219 ppm (o-DCBz)

Solvent: o-Dichlorobenzene-d4

Sample concentration: 1 wt/vol %

Sample tube: 5 mmφ

(¹³C-NMR Measuring Conditions)

Apparatus: Bruker Avance NEO 600

Observation nucleus: ¹³C

Observation frequency: 150.91 MHz

Pulse program: zgpg30

Pulse delay time: 5 sec

Number of scans: 24,000 or 12,800

Measuring temperature: 130° C.

Chemical shift reference: 132.39 ppm (o-DCBz)

Solvent: o-Dichlorobenzene-d4

Sample concentration: 10 wt/vol %

Sample tube: 5 mmφ

For the separator in the crosslinked state, measurement can be performedby NMR in the same manner as FIG. 13 after the pretreatment describedabove (not shown).

As shown in FIGS. 11 to 13 , the ¹H and/or ¹³C NMR measurement allowsthe amount of silane unit modification and the amount of polyolefinalkyl group modification in the silane-modified polyolefin to beconfirmed for a polyolefin starting material, and allows thesilane-modified polyolefin contained in the separator to be determined(—CH₂—Si: ¹H, 0.69 ppm, t; ¹³C, 6.11 ppm, s).

[Combination of Microporous Membrane and Inorganic Porous Layer]

A combination of a silane-modified polyolefin-containing microporousmembrane and an inorganic porous layer will tend to provide both ashutdown function at lower temperatures than 150° C. and membranerupture properties at relatively high temperature, and to improve theelectricity storage device cycle characteristics and battery nailpenetration safety. Since the silane-modified polyolefin in themicroporous membrane has a silane crosslinking property, presumablysilane crosslinking can result in increased viscosity of the resin inthe microporous membrane, and therefore when compressive force isapplied between the electrodes during a period of abnormal hightemperature of the separator-containing electricity storage device, thecrosslinked high-viscosity resin is less likely to flow into theinorganic layer (that is, integration is less likely), and clearancebetween the electrodes can be adequately ensured and shorting of thebattery can be inhibited.

[Inorganic Porous Layer]

The inorganic porous layer is a layer comprising inorganic particles anda resin binder, and optionally it may further comprise a dispersingagent that disperses the inorganic particles in the binder resin.

The thickness of the inorganic porous layer is preferably 0.5 μm to 10μm, 0.5 μm to 7 μm, 0.5 μm to 5 μm or 0.5 μm to 4 μm, from the viewpointof the ion permeability of the separator, and the charge-dischargecapacity or cycle stability of the electricity storage device. Thethickness of the inorganic porous layer can be determined by the methoddescribed in the Examples.

[Second Porous Layer (Layer B)]

Layer B comprises inorganic particles. Layer B may also comprise a resinbinder. When layer B comprises inorganic particles and a resin binder,layer B may be an inorganic porous layer as described above. Layer B mayalso comprise components other than inorganic particles and a resinbinder.

(Thickness of Layer B)

The thickness (TB) of layer B is preferably 0.2 μm or greater and morepreferably 0.5 μm or greater. If the thickness (TB) is 0.5 μm or greaterthe mechanical strength will tend to be further increased. The thickness(TB) is also preferably smaller than 22 μm, more preferably 20 μm orsmaller and even more preferably 15 μm or smaller. If the thickness (TB)is 30 μm or smaller, the volume of the electricity storage deviceoccupied by the separator will be reduced, which will tend to beadvantageous from the viewpoint of increasing the capacity of theelectricity storage device. It is also preferred from the viewpoint ofpreventing excessive increase in the air permeability of the separator.The thickness (TB) may be set to 0.50 μm or greater, 0.80 μm or greateror 1.00 μm or greater, or set to smaller than 22.00 μm, 20.00 μm orsmaller or 15.00 μm or smaller, for example.

The thickness (TB) can be measured by the method described in theExamples, and it can be controlled by varying the coating amount of thecoating solution (slurry) used to form layer B.

When layer B is a single layer, the thickness of layer B is treated asthe “thickness (TB)”. When layer B is multi-layered, the total thicknessof the multiple layers of layer B is treated as the “thickness (TB)”.

When layer B is disposed on both one and the other side of layer A, thetotal thickness of the layer B disposed on the one side and the layer Bdisposed on the other side is treated as the “thickness (TB)”.

(Inorganic Particles)

Examples for the inorganic particles include inorganic oxides(oxide-based ceramics) such as alumina (Al₂O₃), silica, titania,zirconia, magnesia, ceria, yttria, zinc oxide and iron oxide; inorganicnitrides (nitride-based ceramics) such as silicon nitride, titaniumnitride and boron nitride; ceramics such as silicon carbide, calciumcarbonate, magnesium sulfate, aluminum sulfate, aluminum hydroxide,aluminum hydroxide oxide (AlO(OH)), potassium titanate, talc, kaolinite,dickite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite,mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesiumsilicate, diatomaceous earth and quartz sand; and glass fibers. Thesemay be used alone, or two or more may be used in combination.

From the viewpoint of ensuring heat resistance, the amount of inorganicparticles is preferably 5 weight % or greater or 20 weight % or greater,and more preferably 30 weight % or greater, based on the total weight ofthe inorganic porous layer or layer B. The amount of inorganic particlesmay be set to 50 weight % or greater, greater than 80 weight % or 85weight % or greater, based on the total weight of the inorganic porouslayer or layer B. The amount of inorganic particles is also preferablyno greater than 99.9 weight %, and more preferably no greater than 99.5weight % or no greater than 99 weight %.

The amount of inorganic particles may be set 20.00 weight % or greater,30.00 weight % or greater, 50.00 weight % or greater, greater than 80.00weight % or 85.00 weight % or greater, and also set to no greater than99.90 weight % or 99.50 weight %.

The form of the inorganic particles may be tabular, scaly, needle-like,columnar, spherical, polyhedral, fusiform or aggregated (block-shaped).Inorganic particles with these shapes may also be combined for use.

The number-mean particle size of the inorganic particles is preferably0.01 μm or greater, 0.1 μm or greater, 0.3 μm or greater or 0.5 μm orgreater. The number-mean particle size is also preferably no greaterthan 10.0 μm, no greater than 9.0 μm, no greater than 6.0 μm or nogreater than 2.5 μm, more preferably no greater than 2.0 μm and evenmore preferably no greater than 1.5 μm, for example. Adjusting thenumber-mean particle size of the inorganic particles to within thisrange is preferred from the viewpoint of increasing the safety duringshort circuiting. The method of adjusting the number-mean particle sizeof the inorganic particles may be a method of pulverizing the inorganicparticles using a suitable pulverizing apparatus such as a ball mill,bead mill or jet mill.

The particle size distribution of the inorganic particles is preferably0.02 μm or greater, more preferably 0.05 μm or greater and even morepreferably 0.1 μm or greater, as the minimum particle size. The maximumparticle size is preferably no greater than 20 μm, more preferably nogreater than 10 μm and even more preferably no greater than 7 μm. Themaximum particle size/mean particle size ratio is preferably no greaterthan 50, more preferably no greater than 30 and even more preferably nogreater than 20. Adjusting the particle size distribution of theinorganic particles to within this range is preferred from the viewpointof inhibiting heat shrinkage at high temperature. Multiple particle sizepeaks may also be present between the maximum particle size and minimumparticle size. The method of adjusting the particle size distribution ofthe inorganic particles may be, for example, a method of pulverizing theinorganic filler using a ball mill, bead mill or jet mill to adjust themto the desired particle size distribution, or a method of preparingmultiple fillers with different particle size distributions and thenblending them.

(Resin Binder)

The resin binder comprises a resin that binds together the inorganicparticles. The glass transition temperature (Tg) of the resin binder ispreferably −50° C. to 100° C. and more preferably −35° C. to 95° C.,from the viewpoint of ensuring the binding property with the inorganicparticles, and ensuring stability of the inorganic porous layer or layerB, during the production process for the separator, the productionprocess for the electricity storage device or the charge-dischargeprocess.

The glass transition temperature is determined from a DSC curve obtainedby differential scanning calorimetry (DSC). Specifically, the value usedfor the glass transition temperature may be the temperature at theintersection between a straight line extending the low-temperature endbaseline in the DSC curve toward the high-temperature end, and thetangent line at the inflection point in the stepwise change region ofglass transition. More specifically, it may be determined by the methoddescribed in the Examples. Moreover, the “glass transition” refers tothe value when a change in heat quantity accompanying the change instate of a polymer test piece in DSC occurs at the endothermic end. Thechange in heat quantity is observed in the form of a stepwise change inthe DSC curve. A “stepwise change” is a portion of the DSC curve movingaway from the previous low-temperature end baseline and toward a newhigh-temperature end baseline. A combination of a stepwise change and apeak is also included in the concept of “stepwise change”. The“inflection point” is the point at which the slope of the DSC curve ismaximum in the stepwise change region. If the exothermic end in thestepwise change region is defined as the top end, then this representsthe point where the upwardly convex curve changes to a downwardly convexcurve. The term “peak” refers to a portion of the DSC curve that movesaway from the low-temperature end baseline and then returns to the samebaseline. The term “baseline” refers to the DSC curve in the temperaturezone where no transition or reaction takes place in the test piece.

Examples for the resin binder include the following 1) to 7), forexample. These may be used alone, or two or more may be used incombination.

1) Polyolefins: Polyethylene, polypropylene, ethylene-propylene rubberand modified forms of these;

2) Conjugated diene-based polymers: For example, styrene-butadienecopolymers and their hydrogenated forms, acrylonitrile-butadienecopolymers and their hydrogenated forms andacrylonitrile-butadiene-styrene copolymers and their hydrogenated forms;

3) Acrylic-based polymers: For example, methacrylic acid ester-acrylicacid ester copolymers, styrene-acrylic acid ester copolymers andacrylonitrile-acrylic acid ester copolymers;

4) Polyvinyl alcohol-based resins: For example, polyvinyl alcohol andpolyvinyl acetate;

5) Fluorine-containing resins: For example, PVdF,polytetrafluoroethylene, vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene copolymer andethylene-tetrafluoroethylene copolymer;

6) Cellulose derivatives: For example, ethyl cellulose, methylcellulose, hydroxyethyl cellulose and carboxymethyl cellulose; and

7) Polymers that are resins with a melting point and/or glass transitiontemperature of 180° C. or higher, or without a melting point but havinga decomposition temperature of 200° C. or higher: For example,polyphenylene ethers, polysulfones, polyethersulfones, polyphenylenesulfides, polyetherimides, polyamideimides, polyamides and polyesters.

These types of resin binders can be obtained by known production methodssuch as emulsion polymerization or solution polymerization, using anydesired monomers as the starting materials. The polymerization is notrestricted in terms of the polymerization temperature, the pressureduring polymerization, the method of adding the monomers and theadditives used (polymerization initiator, molecular weight modifier andpH regulator, etc.).

The amount of resin binder is 0.5 weight % or greater or 1.0 weight % orgreater, for example, and no greater than 50 weight % or no greater than30 weight %, for example, based on the total weight of the inorganicporous layer or layer B. Since layer B has the resin binder as anoptional component as mentioned above, the amount of resin binder inlayer B may be less than 20 weight %, 15 weight % or less or 0 weight %based on the total weight of layer B. If the amount of resin binder inlayer B is reduced, it will be possible to increase the amount ofinorganic particles added to layer B by that amount.

(Dispersing Agent)

The dispersing agent is adsorbed onto the surfaces of the inorganicparticles in the slurry to form the inorganic porous layer or layer B,thus stabilizing the inorganic particles by electrostatic repulsion andthe like, and examples thereof include polycarboxylic acid salts,sulfonic acid salts, polyoxyethers and surfactants. The inorganic porouslayer or layer B may also include other components commonly added toaqueous coating materials in addition to the components mentioned above,within the range of the aforementioned effect. Such other componentsinclude, but are not limited to, thickeners, membrane-forming aids,plasticizers, crosslinking agents, cryoprotectants, antifoaming agents,dyes, antiseptic agents, ultraviolet absorbers and light stabilizers,for example. Such other components may be used alone, or two or more maybe used in combination.

(Additives)

The microporous membrane, inorganic porous layer, layer A and/or layer Bmay also include known additives as necessary. Examples of suchadditives include organometallic catalysts (dehydrating condensationcatalysts); plasticizers; phenol-based, phosphorus-based andsulfur-based antioxidants; metal soaps such as calcium stearate and zincstearate; thickeners; membrane-forming aids; crosslinking agents;cryoprotectants; antifoaming agents; antiseptic agents; ultravioletabsorbers; light stabilizers; antistatic agents; anti-fogging agents;dyes; and color pigments.

Layer B may also include a crosslinking agent. The crosslinking agentmay include a functional group that reacts with the inorganic particles.

<Physical Properties of Separator>

When the separator is to be used in a relatively high-capacity lithiumion secondary battery, the membrane thickness of the separator as awhole is preferably no greater than 25 μm, more preferably no greaterthan 22 μm or no greater than 20 μm, even more preferably no greaterthan 18 μm and most preferably no greater than 16 μm. If the membranethickness of the separator is no greater than 25 μm, the ionpermeability will tend to be further increased. The lower limit for themembrane thickness of the separator as a whole may be 1.0 μm or greater,3.0 μm or greater, 4.0 μm or greater, 6.0 μm or greater or 7.5 μm orgreater, for example.

The air permeability of the separator is preferably 50 seconds/100 cm³to 400 seconds/100 cm³, more preferably 75 seconds/100 cm³ to 275seconds/100 cm³ and even more preferably 100 seconds/100 cm³ to 200seconds/100 cm³. This is preferred because the separator will havesuitable mechanical strength so long as the air permeability is 50seconds/100 cm³ or greater, and will have an improved batterycharacteristic from the viewpoint of permeability if the airpermeability is 400 seconds/100 cm³ or less.

[Electricity Storage Device Assembly Kit]

According to another aspect of the invention there is provided anelectricity storage device assembly kit comprising the separator for anelectricity storage device described above. The electricity storagedevice assembly kit comprises the following two elements:

(A) an exterior body housing electrodes and a laminated stack or woundbody of the separator for an electricity storage device according to anyof the embodiments described above; and

(B) a container housing a nonaqueous electrolyte solution. During use ofthe electricity storage device assembly kit, the separator in element(A) is contacted with the nonaqueous electrolyte solution in element(B), thereby contacting the electrolyte solution and the laminated stackor wound body inside the exterior body and/or continuously carrying outcharge-discharge cycling of the assembled electricity storage device, toform a crosslinked structure in the separator, so that an electricitystorage device having both safety and output can be formed.

While it is not our intention to be limited to any particular theory, itis possible that when the electrolyte or electrolyte solution contactswith the electrodes and/or charge-discharge of the electricity storagedevice is carried out, the substance that is responsible for catalyticaction during the crosslinking reaction or substances with functionalgroups that form part of the crosslinked structure, being present in theelectrolyte solution, on the exterior body inner walls or on theelectrode surfaces, dissolve into the electrolyte solution and evenlyswell and diffuse into the amorphous portions of the polyolefin, therebyhomogeneously promoting crosslinking reaction of theseparator-containing laminated stack or wound body. The substanceresponsible for catalytic action during the crosslinking reaction may bein the form of an acid solution or membrane, and when the electrolyteincludes lithium hexafluorophosphate (LiPF₆), it may be hydrofluoricacid (HF) or a fluorine-containing organic substance derived fromhydrofluoric acid (HF). Substances with functional groups that form partof the crosslinked structure may include the compound with functionalgroup A and/or B described above, or the electrolyte solution itself, orvarious additives.

From the viewpoint of promoting crosslinking reaction of the separator,the electrolyte in the nonaqueous electrolyte solution housed in element(2) may be a fluorine (F)-containing lithium salt such as LiPF₆ or anelectrolyte having an unshared electron pair such as LiN(SO₂CF₃)₂ orLiSO₃CF₃, which generates HF, or it may be LiBF₄ or LiBC₄O₈ (LiBOB).

From the viewpoint of promoting crosslinking reaction of the separator,the electricity storage device assembly kit may be provided with aseparate container as an accessory (or element (C)), which houses acatalyst to promote the crosslinking reaction, such as a mixture of anorganometallic catalyst and water, an acid solution or a base solution.

[Electricity Storage Device]

The separator described above can be used in an electricity storagedevice. The electricity storage device comprises a positive electrode, anegative electrode, a separator according to this embodiment disposedbetween the positive and negative electrodes, an electrolyte solution,and optionally an additive. When the separator is housed in a deviceexterior body, the functional group-modified polyethylene or functionalgroup graft-copolymerized polyethylene reacts with chemical substancesin the electrolyte solution or additives, forming a crosslinkedstructure, and therefore a crosslinked structure is present in thefabricated electricity storage device. The functional group-modifiedpolyethylene and functional group graft-copolymerized polyethylene arenot limited, and they may be derived from the polyolefin of themicroporous membrane or derived from a polyolefin that has been modifiedduring the production process for the microporous membrane.

Specifically, the electricity storage device may be a lithium battery,lithium secondary battery, lithium ion secondary battery, sodiumsecondary battery, sodium ion secondary battery, magnesium secondarybattery, magnesium ion secondary battery, calcium secondary battery,calcium ion secondary battery, aluminum secondary battery, aluminum ionsecondary battery, nickel hydrogen battery, nickel cadmium battery,electrical double layer capacitor, lithium ion capacitor, redox flowbattery, lithium sulfur battery, lithium-air battery or zinc airbattery, for example. Preferred among these, from the viewpoint ofpracticality, are a lithium battery, lithium secondary battery, lithiumion secondary battery, nickel hydrogen battery or lithium ion capacitor,with a lithium battery or lithium ion secondary battery being morepreferred.

The additive may be a dehydrating condensation catalyst, a metal soapsuch as calcium stearate or zinc stearate, or an ultraviolet absorber,light stabilizer, antistatic agent, anti-fogging agent or color pigment,for example.

[Lithium Ion Secondary Battery]

A lithium ion secondary battery is a battery employing a lithiumtransition metal oxide such as lithium cobaltate or a lithium cobaltcomposite oxide as the positive electrode, a carbon material such asgraphite or graphite as the negative electrode, and an organic solventcontaining a lithium salt such as LiPF₆ as the electrolyte solution. Theelectrolyte solution described above may also be used in a lithium ionsecondary battery for the electricity storage device assembly kit.

During charge and discharge of the lithium ion secondary battery,ionized lithium reciprocates between the electrodes. The separator isdisposed between the electrodes since the ionized lithium must migratebetween the electrodes relatively rapidly while contact between theelectrodes is inhibited.

<Method for Producing Separator for Electricity Storage Device>

Another aspect of the invention is a method for producing a separatorfor an electricity storage device. The method for producing theseparator may comprise a step of producing the microporous membrane orlayer A, and optionally a step of producing an inorganic porous layer onthe microporous membrane or a step of producing layer B on layer A. Thematerials used in the method for producing the separator may be thosementioned for the first to tenth embodiments, unless otherwisespecified.

Eleventh Embodiment

The method for producing a separator according to the eleventhembodiment will now be explained for a microporous membrane (flatmembrane), with the understanding that other forms in addition to flatmembranes are not excluded. The method for producing a microporousmembrane according to the eleventh embodiment comprises the followingsteps:

(1) a sheet-forming step;

(2) a stretching step;

(3) a porous body-forming step; and

(4) a heat treatment step. Layer A described above can be formed bycarrying out steps (1) to (4).

The method for producing a separator according to the eleventhembodiment may optionally include, in addition to steps (1) to (4), alsothe following steps:

(8B) a coating step in which an inorganic porous layer that includesinorganic particles and a resin binder is formed on at least one surfaceof the heat-treated porous body to form a silane-crosslinking precursor;

(9) an assembly step in which the electrodes, the laminated stack of thesilane-crosslinking precursor or its wound body and the nonaqueouselectrolyte solution are housed in an exterior body, and thesilane-crosslinking precursor is contacted with the nonaqueouselectrolyte solution. According to the eleventh embodiment, in step (8B)the inorganic porous layer is coated onto the microporous membrane whichmaintains its silane crosslinkability, after which in step (9) theseparator and electrolyte solution are contacted inside the electricitystorage device, and therefore the stress resistance of the electricitystorage device and the separator in it is improved, and cycle stabilityand safety can be achieved for the electricity storage device.

The method for producing the microporous membrane of the eleventhembodiment may optionally include a kneading step before thesheet-forming step (1) and/or a winding and slitting step after the heattreatment step (3), but preferably it does not include a silanecrosslinking treatment step from the viewpoint of maintaining silanecrosslinkability until contact with the electrolyte solution. The silanecrosslinking treatment step will generally be a step in which the targetof treatment that contains a silane-modified polyolefin is contactedwith the mixture of an organometallic catalyst and water, or is immersedin a base solution or acid solution, for silane dehydration condensationreaction to form oligosiloxane bonds.

The metal in an organometallic catalyst may be one or more selected fromthe group consisting of scandium, titanium, vanadium, copper, zinc,aluminum, zirconium, palladium, gallium, tin and lead, for example. Theorganometallic catalyst may be di-butyltin-di-laurate,di-butyltin-di-acetate, di-butyltin-di-octoate, or the like, which areknown to overwhelmingly accelerate the reaction rate by the reactionmechanism proposed by Weij et al. (F. W. van. der. Weij: Macromol.Chem., 181, 2541, 1980). In recent years it is known that, in order toavoid damage to the environment and human health by organic tin, theLewis functions of chelate complexes of copper and/or titanium can beenutilized and combined with organic bases to promote reaction formingsiloxane bonds between alkoxysilyl groups, similar to organic tincomplexes.

The base solution may have a pH of higher than 7 and may include alkalihydroxide metals, alkaline earth metal hydroxides, alkali metalcarbonates, alkali metal phosphates, ammonia or amine compounds, forexample. Of these, alkali metal hydroxides and alkaline earth metalhydroxides are preferred, alkali metal hydroxides are more preferred andsodium hydroxide is even more preferred, from the viewpoint of theelectricity storage device safety and silane crosslinkability.

The acid solution is a pH of below 7 and may include inorganic acids ororganic acids, for example. Preferred acids are hydrochloric acid,sulfuric acid, carboxylic acids or phosphoric acids.

In the kneading step, a kneading machine may be used for kneading of thesilane-modified polyolefin, for this embodiment, and optionally aplasticizer or inorganic material and another polyolefin. From theviewpoint of inhibiting generation of resin aggregates during theproduction process and maintaining silane crosslinkability until contactwith the electrolyte solution, a master batch resin containing adehydrating condensation catalyst is preferably not added to the kneadedproduct.

The plasticizer is not particularly restricted, and examples includeorganic compounds that can form homogeneous solutions with polyolefinsat temperatures below their boiling points. More specifically, theseinclude decalin, xylene, dioctyl phthalate, dibutyl phthalate, stearylalcohol, oleyl alcohol, decyl alcohol, nonyl alcohol, diphenyl ether,n-decane, n-dodecane and paraffin oil. Paraffin oil and dioctylphthalate are preferred among these. A plasticizer may be used alone, ortwo or more may be used in combination. The proportion of theplasticizer is not particularly restricted, but from the viewpoint ofthe porosity of the obtained microporous membrane it is preferably 20weight % or greater, and from the viewpoint of the viscosity during meltkneading it is preferably no greater than 90 weight %, as necessary,with respect to the total weight of the polyolefin and silane-modifiedpolyolefin.

The sheet-forming step is a step in which the obtained kneaded blend ora mixture of the silane-modified polyolefin, polyethylene andplasticizer is extruded, cooled to solidification, and cast into a sheetform to obtain a sheet. The sheet forming method is not particularlyrestricted, and may be a method of compressed-cooling solidification ofa molten mixture obtained by melt kneading and extrusion. The coolingmethod may be a method of direct contact with a cooling medium such ascold air or cooling water; or a method of contact with arefrigerant-cooled roll and/or a pressing machine, with a method ofcontact with a refrigerant-cooled roll and/or a pressing machine beingpreferred for superior membrane thickness control.

From the viewpoint of resin aggregates in the separator, and the maximuminternal heat release rate, the weight ratio of the silane-modifiedpolyolefin and polyethylene in the sheet-forming step (silane-modifiedpolyolefin weight/polyethylene weight) is preferably 0.05/0.95 to0.4/0.6 and more preferably 0.06/0.94 to 0.38/0.62.

From the viewpoint of exhibiting low temperature shutdown at 150° C. andbelow and rupture resistance at high temperatures of 180 to 220° C.,while also inhibiting thermal runaway during destruction of theelectricity storage device to improve safety, preferably thesilane-modified polyolefin in the sheet-forming step is not a masterbatch resin that contains a dehydrating condensation catalyst thatcrosslinks the silane-modified polyolefin prior to the sheet-formingstep.

The stretching step is a step in which the plasticizer and/or inorganicmaterial is extracted from the obtained sheet as necessary, and thesheet is further subjected to stretching in one or more axialdirections. The method of stretching the sheet may be MD uniaxialstretching with a roll stretcher, TD uniaxial stretching with a tenter,sequential biaxial stretching with a combination of a roll stretcher andtenter, or a tenter and tenter, or simultaneous biaxial stretching witha biaxial tenter or inflation molding. Simultaneous biaxial stretchingis preferred from the viewpoint of obtaining a more homogeneousmembrane. The total area increase is preferably 8-fold or greater, morepreferably 15-fold or greater and even more preferably 20-fold orgreater or 30-fold or greater, from the viewpoint of membrane thicknesshomogeneity, and balance between tensile elongation, porosity and meanpore size. If the total area increase is 8-fold or greater, it will tendto be easier to obtain high strength and a satisfactory thicknessdistribution. The area increase is also no greater than 250-fold fromthe viewpoint of preventing rupture.

The porous body-forming step is a step in which the plasticizer isextracted from the stretched sheet after the stretching step to formpores in the stretched sheet. The method of extracting the plasticizeris not particularly restricted, and may be a method of immersing thestretched sheet in an extraction solvent or a method of showering thestretched sheet with an extraction solvent, for example. The extractionsolvent used is not particularly restricted, but it is preferably onethat is a poor solvent for the polyolefin and a good solvent for theplasticizer and/or inorganic material, and that has a boiling point thatis lower than the melting point of the polyolefin. Such extractionsolvents are not particularly restricted and include hydrocarbons suchas n-hexane and cyclohexane; halogenated hydrocarbons such as methylenechloride, 1,1,1-trichloroethane and fluorocarbon-based compounds;alcohols such as ethanol and isopropanol; ketones such as acetone and2-butanone; and alkali water. An extraction solvent may be used alone,or two or more may be used in combination.

The heat treatment step is a step in which, after the stretching step,the plasticizer is also extracted from the sheet as necessary and heattreatment is further carried out to obtain a microporous membrane. Themethod of heat treatment is not particularly restricted, and forexample, it may be a heat setting method in which a tenter and/or rollstretcher is utilized for stretching and relaxation procedures. Arelaxation procedure is a procedure of shrinking carried out at aprescribed temperature and relaxation factor, in the machine direction(MD) and/or transverse direction (TD) of the membrane. The relaxationfactor is the value of the MD dimension of the membrane after therelaxation procedure divided by the MD dimension of the membrane beforethe procedure, or the value of the TD dimension after the relaxationprocedure divided by the TD dimension of the membrane before theprocedure, or the product of the relaxation factor in the MD and therelaxation factor in the TD, when both the MD and TD have been relaxed.

[Inorganic Porous Layer Coating Step]

The coating step (8B) for the inorganic porous layer is a step in whichan inorganic porous layer comprising inorganic particles and a resinbinder is formed on at least one surface of the microporous membraneobtained as described above. The coating step (8B) may be carried outwhile maintaining the silane crosslinkability of the silane-modifiedpolyolefin.

Layer B described above can be formed by carrying out the coating step(8B). The method used to form layer B may be a known production method.The method of fabricating a laminated stack comprising layer A and layerB may be, for example, a method of coating an inorganicparticle-containing slurry onto layer A, a method of layering andextruding the starting material for layer B and the starting materialfor layer A by a co-extrusion method, or a method of separatelypreparing layer A and layer B and then attaching them together.

The inorganic porous layer can be formed, for example, by coating atleast one surface of the microporous membrane with a slurry containinginorganic particles, a resin binder, water or an aqueous solvent (forexample, a mixture of water and an alcohol) and optionally a dispersingagent. The inorganic particles, resin binder and dispersing agent may beas described above for the first to tenth embodiments.

The solvent in the slurry is preferably one that can uniformly andstably disperse or dissolve the inorganic particles. Examples of suchsolvents include N-methylpyrrolidone (NMP), N,N-dimethylformamide,N,N-dimethylacetamide, water, ethanol, toluene, hot xylene, methylenechloride and hexane.

The method of preparing the inorganic particle-containing slurry may be,for example, a mechanical stirring method using a ball mill, bead mill,planetary ball mill, vibrating ball mill, sand mill, colloid mill,attritor, roll mill, high-speed impeller disperser, disperser,homogenizer, high-speed impact mill, ultrasonic disperser or stirringblade.

Examples for the method of coating the inorganic particle-containingslurry include gravure coater methods, small-diameter gravure coatermethods, reverse roll coater methods, transfer roll coater methods, kisscoater methods, dip coater methods, knife coater methods, air doctorcoater methods, blade coater methods, rod coater methods, squeeze coatermethods, cast coater methods, die coater methods, screen printingmethods and spray coating methods.

The method for removing the solvent from the coated membrane may be amethod of drying at a temperature below the melting point of thematerial forming the microporous membrane, or a method of reducedpressure drying at low temperature. Some of the solvent may be allowedto remain so long as it does not produce any notable effect on thedevice properties.

[Winding/Slitting Step]

The winding step is a step in which the obtained microporous membrane orthe inorganic porous layer-coated microporous membrane is slit ifnecessary and wound onto a prescribed core.

[Electricity Storage Device Assembly Step]

The electricity storage device assembly step is a step in which aseparator precursor that maintains its silane crosslinkability(hereunder also referred to as “silane-crosslinking precursor”) andelectrodes are layered to form a laminated stack, the laminated stack isoptionally wound to form a wound body, and the laminated stack or woundbody and a nonaqueous electrolyte solution are housed in an exteriorbody, causing contact between the silane-crosslinking precursor and thenonaqueous electrolyte solution. The electricity storage device assemblystep allows membrane loss of the microporous membrane to be reduced tomaintain its morphology and can inhibit infiltration of the polyolefinresin from the microporous membrane into the inorganic porous layer,thus improving the stress resistance of the electricity storage deviceor separator.

Since the silane-modified polyolefin is crosslinked either during theelectricity storage device assembly step (9) or after step (9), it issuitable for conventional electricity storage device productionprocesses, while it also produces silane crosslinking reaction of theseparator after production of the electricity storage device, therebyimproving the safety of the electricity storage device.

From the viewpoint of handleability of the electrolyte solution duringthe electricity storage device assembly step, preferably the nonaqueouselectrolyte solution is injected into the exterior body after thelaminated stack or wound body has been housed in the exterior body, orthe laminated stack or wound body is housed in the exterior body afterthe electrolyte solution has been injected into the exterior body.

From the viewpoint of promoting crosslinking reaction of the separator,the electrolyte of the nonaqueous electrolyte solution may be a fluorine(F)-containing lithium salt such as LiPF₆ or an electrolyte having anunshared electron pair such as LiN(SO₂CF₃)₂ or LiSO₃CF₃, which generateshydrofluoric acid (HF), or it may be LiBF₄ or LiBC₄O₈ (LiBOB).

While it is not our intention to be limited to any particular theory, itis conjectured that the methoxysilane-grafted portions are converted tosilanol by trace amounts of water inside the electricity storage device(water present in the members such as the electrodes, separator andelectrolyte solution), resulting in crosslinking reaction and conversionto siloxane bonds. It is possible that when the electrolyte orelectrolyte solution contacts with the electrodes, substances thatproduce catalytic action in the silane crosslinking reaction aregenerated in the electrolyte solution or on the electrode surfaces anddissolve into the electrolyte solution, resulting in uniform swellingand diffusion in the amorphous portions of the polyolefin where thesilane-modified grafts are present, and thus homogeneously promotingcrosslinking reaction of the separator-containing laminated stack orwound body. The substances responsible for catalytic action in thesilane crosslinking reaction may be in the form of an acid solution ormembrane, and when the electrolyte includes lithium hexafluorophosphate(LiPF₆), it may be HF generated by reaction between LiPF₆ and water, ora fluorine-containing organic substance derived from HF.

From the viewpoint of efficient silane crosslinking reaction, it ispreferred to connect lead terminals to the electrodes and to conduct atleast one cycle of charge-discharge, after the laminated stack or woundbody and the nonaqueous electrolyte solution in the exterior body of theelectricity storage device have been housed in the exterior body. As aresult of charge-discharge cycling, presumably, substances responsiblefor catalytic action in the silane crosslinking reaction are produced inthe electrolyte solution or on the electrode surfaces, so that silanecrosslinking reaction is achieved. The cycling charge-discharge can becarried out with a known method and apparatus, and specifically themethod described in the Examples may be employed.

[Method for Producing Electricity Storage Device]

Another aspect of the invention is a method for producing an electricitystorage device.

Twelfth Embodiment

The method for producing an electricity storage device according to thetwelfth embodiment comprises the following steps:

(a) a step of preparing an electricity storage device assembly kit asdescribed above,

(b) a step of contacting the separator in element (1) and the nonaqueouselectrolyte solution in element (2) of the electricity storage deviceassembly kit, to initiate silane crosslinking reaction of thesilane-modified polyolefin,

(c) optionally, a step of connecting lead terminals to the electrodes ofelement (1), and

(d) optionally, a step of carrying out at least one cycle ofcharge-discharge. Steps (a) to (d) can be carried out by a method knownin the technical field, except for using a separator for an electricitystorage device according to this embodiment, and a positive electrode,negative electrode, electrolyte solution, exterior body andcharge-discharge apparatus known in the technical field may be used insteps (a) to (d).

A separator with a longitudinal shape having a width of 10 to 500 mm(preferably 80 to 500 mm) and a length of 200 to 4000 m (preferably 1000to 4000 m) may be produced for step (a). Next, in step (a), laminationmay be carried out in the order: positive electrode-separator-negativeelectrode-separator or negative electrode-separator-positiveelectrode-separator, and the laminate wound into a circular or flatspiral form to obtain a wound body. In steps (b) and (c), the wound bodymay be housed in a device can (for example, a battery can) and anonaqueous electrolyte solution injected to produce an electricitystorage device. The electrodes and the wound body obtained by foldingthe separator may then be placed in a device container (for example, analuminum film) and a nonaqueous electrolyte solution may be injected,thereby producing an electricity storage device.

The wound body may also be pressed during this time. Specifically, theseparator may be stacked and pressed with an electrode having a currentcollector and an active material layer formed on at least one side ofthe current collector.

The pressing temperature is preferably 20° C. or higher, as an exampleof a temperature allowing the adhesion to be effectively exhibited. Fromthe viewpoint of inhibiting blocking or heat shrinkage of the pores inthe separator by hot pressing, the pressing temperature is preferablylower than the melting point of the material in the microporousmembrane, and more preferably no higher than 120° C. The pressingpressure is preferably no higher than 20 MPa from the viewpoint ofinhibiting blocking of the pores of the separator. The pressing time maybe up to 1 second when a roll press is used, or several hours forsurface pressing, but from the viewpoint of productivity it ispreferably no longer than 2 hours.

By this production process it is possible to reduce press back duringpress molding of a wound body comprising the electrodes and theseparator. It is thus possible to inhibit yield reduction in the deviceassembly steps and shorten the production process time.

It is preferred to carry out steps (c) and (d) from the viewpoint ofreliably carrying out silane crosslinking reaction of the separatorafter step (b). As a result of charge-discharge cycling, presumably,substances responsible for catalytic action in the silane crosslinkingreaction are produced in the electrolyte solution or on the electrodesurfaces, so that silane crosslinking reaction is achieved.

For example, when the method for producing layer A described above inthe method for producing the separator does not include a silanecrosslinking treatment step, the separator may be contacted with thenonaqueous electrolyte solution to actively promote the crosslinkingreaction. While it is not our intention to be limited to any particulartheory, it is conjectured that the silane-modified graft portions areconverted to silanol by trace amounts of water inside the electricitystorage device (water present in trace amounts in the electrodes,separator and nonaqueous electrolyte solution),

resulting in crosslinking reaction and conversion to siloxane bonds.When the nonaqueous electrolyte solution contacts with the electrodes,presumably the substances responsible for catalytic action in the silanecrosslinking reaction are produced in the nonaqueous electrolytesolution and on the electrode surfaces. It is possible that thesubstances responsible for catalytic action in the silane crosslinkingreaction dissolve into the nonaqueous electrolyte solution, resulting inuniform swelling and diffusion in the amorphous portions of thepolyolefin where the silane-modified grafts are present, and thushomogeneously promoting crosslinking reaction of theseparator-containing laminated stack or wound body.

The substances responsible for catalytic action in the silanecrosslinking reaction may be in the form of an acid solution ormembrane. When the electrolyte includes lithium hexafluorophosphate(LiPF₆), the LiPF₆ reacts with water, and hydrofluoric acid (HF)generated by the reaction or fluorine-containing organic substancesderived from hydrofluoric acid (HF) are treated as substancesresponsible for catalytic action in the silane crosslinking reaction(compounds generated inside the electricity storage device).

Thirteenth Embodiment

The thirteenth embodiment is a method for producing an electricitystorage device using a separator that includes a polyolefin having oneor two or more different functional groups, the method comprising thefollowing step:

a crosslinking step in which (1) condensation reaction is carried outbetween the functional groups, (2) the functional groups are reactedwith a chemical substance inside the electricity storage device, or (3)the functional groups of the polyolefin are reacted with different typesof functional groups, to form a crosslinked structure.

The crosslinking step can be carried out in the same manner as thereaction for formation of the crosslinked structure in the separator,described above. Since the crosslinking step can also be carried oututilizing compounds in the electricity storage device or the surroundingenvironment of the device, it is possible to employ mild conditions suchas a temperature of 5° C. to 90° C. and/or ambient atmosphere, withoutrequiring excessive conditions such as an electron beam or a hightemperature of 100° C. or above.

By carrying out the crosslinking step during the production process forthe electricity storage device, it is possible to eliminate formation ofa crosslinked structure either during or immediately after the processof forming the separator membrane, which can alleviate or eliminatestress strain following fabrication of the electricity storage device,and/or the separator can be imparted with a crosslinked structurewithout using relatively high energy such as photoirradiation orheating, thus allowing crosslinking unevenness, non-molten resinaggregate generation and environmental load to be reduced.

By reaction of functional groups with a chemical substance inside theelectricity storage device (2) or reaction of the functional groups ofthe polyolefin with other types of functional groups (3) in thecrosslinking step, a crosslinked structure can be formed not only withinthe separator but also between the separator and the electrodes orbetween the separator and the solid electrolyte interface (SEI), thusincreasing the strength between multiple members of the electricitystorage device.

Since the silane-modified polyolefin is crosslinked when the separatordescribed above contacts with the electrolyte solution, it is suitablefor conventional electricity storage device production processes, whileit also produces silane crosslinking reaction after production of theelectricity storage device, thereby improving the safety of theelectricity storage device.

EXAMPLES

The present invention will now be explained in greater detail byexamples and comparative examples, with the understanding that theinvention is not limited to the examples so long as its gist ismaintained. The physical properties in the examples were measured by thefollowing methods.

<Weight-Average Molecular Weight>

Standard polystyrene was measured using a Model ALC/GPC 150C™ by WatersCo. under the following conditions, and a calibration curve was drawn.The chromatogram for each polymer was also measured under the sameconditions, and the weight-average molecular weight of each polymer wascalculated by the following method, based on the calibration curve.

Column: GMH₆-HT™ (2)+GMH₆-HTL™ (2), by Tosoh Corp.

Mobile phase: o-Dichlorobenzene

Detector: differential refractometer

Flow rate: 1.0 ml/min

Column temperature: 140° C.

Sample concentration: 0.1 wt %

(Weight-Average Molecular Weight of Polyethylene)

Each molecular weight component in the obtained calibration curve wasmultiplied by 0.43 (polyethylene Q factor/polystyrene Qfactor=17.7/41.3), to obtain a molecular weight distribution curve interms of polyethylene, and the weight-average molecular weight wascalculated.

(Weight-Average Molecular Weight of Resin Composition)

The weight-average molecular weight was calculated in the same manner asfor polyethylene, except that the Q factor value for the polyolefin withthe largest weight fraction was used.

<Viscosity-Average Molecular Weight (Mv)>

The limiting viscosity [η] (dl/g) at 135° C. in a decalin solvent wasdetermined based on ASTM-D4020. The My of polyethylene was calculated bythe following formula.[η]=6.77×10⁻⁴ Mv^(0.67)<Melt Mass-Flow Rate (MFR) (g/10 Min)>

A melt mass-flow rate measuring device by Toyo Seiki Co., Ltd. (MeltIndexer F-F01) was used to determine the weight of the resin extrudedfor 10 minutes under conditions of 190° C., 2.16 kg pressure, as the MFRvalue.

<Measurement of Glass Transition Temperature>

An appropriate amount of the resin sample-containing aqueous dispersion(solid content=38 to 42 wt %, pH=9.0) was placed in an aluminum pan anddried for 30 minutes with a hot air drier at 130° C. to obtain a drymembrane. Approximately 17 mg of the dried membrane was packed into anmeasuring aluminum container, and DSC and DDSC curves were obtainedusing a DSC measuring apparatus (model DSC6220 by Shimadzu Corp.) undera nitrogen atmosphere. The measuring conditions were as follows.

Stage 1 heating program: Start=70° C., temperature increase at 15°C./min. Temperature maintained for 5 minutes after reaching 110° C.

Stage 2 cooling program: Temperature decrease from 110° C. at 40°C./min. Temperature maintained for 5 minutes after reaching −50° C.

Stage 3 heating program: Temperature decrease from −50° C. to 130° C. at15° C./min. Recording of DSC and DDSC data during stage 3 temperatureincrease.

The intersection between the baseline (an extended straight line towardthe high-temperature end from the baseline of the obtained DSC curve)and the tangent line at the inflection point (the point where theupwardly convex curve changed to a downwardly convex curve) was recordedas the glass transition temperature (Tg).

<Membrane Thickness (μm)>

A KBM™ microthickness meter by Toyo Seiki Co., Ltd. was used to measurethe membrane thickness of the microporous membrane or separator at roomtemperature (23±2° C.) and 60% relative humidity. Specifically, themembrane thickness was measured at 5 points at approximately equalintervals across the entire width in the TD direction, and the averagevalue was calculated. The thickness of the inorganic porous layer can becalculated by subtracting the thickness of the microporous membrane fromthe thickness of the separator comprising the microporous membrane andthe inorganic porous layer.

<Layer a Thickness (TA), Layer B Thickness (TB)>

A KBM™ microthickness meter by Toyo Seiki Co., Ltd. was used to measurethe thickness (TA) of layer A at room temperature (23±2° C.) and 60%relative humidity. Specifically, the membrane thickness was measured at5 points at approximately equal intervals across the entire width in theTD, and the average value was calculated. The thickness of the laminatedstack including layer A and layer B was obtained by the same method. Thethickness (TA) of layer A was subtracted from the thickness of theobtained laminated stack to obtain the thickness (TB) of the layer B.

The thickness of the obtained laminated stack was treated as the totalthickness (TA+TB) of layer A and layer B. The thickness (TA) was dividedby the thickness (TB) to obtain the thickness ratio (TA/TB).

<Porosity (%)>

(i) Calculation from Density of Mixed Composition

A 10 cm×10 cm-square sample was cut out from the microporous membrane,and its volume (cm³) and mass (g) were determined and used together withthe density (g/cm³) by the following formula, to obtain the porosity.The density value used for the mixed composition was the valuedetermined by calculation from the densities of the starting materialsused and their mixing ratio.Porosity (%)=(Volume−(mass/density of mixed composition))/volume×100(ii) Calculation from Membrane Density

Alternatively, the porosity of the microporous membrane may becalculated by the following formula from the volume, mass and membranedensity (g/cm³).Porosity (%)=(Volume−(mass/membrane density of mixedcomposition))/volume×100

The membrane density, for the purpose of the present disclosure, is thevalue measured according to the density gradient tube method D describedin JIS K7112(1999).

(iii) Porosity of Layer A

A 10 cm×10 cm-square sample was cut out from layer A, and its volume(cm³) and mass (g) were determined and used together with the density(g/cm³) in the following formula, to obtain the porosity. The densityvalue used for the mixed composition was the value determined bycalculation from the densities of the starting materials used and theirmixing ratio.Porosity (%)=(Volume−(mass/density of mixed composition))/volume×100<Air Permeability (Sec/100 cm³)>

The air permeability of the sample or layer A was measured with a Gurleyair permeability tester (G-B2™ by Toyo Seiki Kogyo Co., Ltd.), accordingto HS P-8117(2009).

<Puncture Strength of Layer A>

Using a Handy Compression Tester KES-G5 (model name) by Kato Tech Corp.,layer A was anchored with a specimen holder having an opening diameterof 11.3 mm. Next, the center section of the anchored layer A wassubjected to a puncture test with a needle having a tip curvature radiusof 0.5 mm, at a puncture speed of 2 mm/sec and a 25° C. atmosphere, tomeasure the maximum puncture load. The value of the maximum punctureload per 20 μm thickness was recorded as the puncture strength (gf/20μm). When the thermoplastic polymer is only present on one side of thebase material, the needle may be used for piercing from the side wherethe thermoplastic polymer is present.

<Quantification of Resin Aggregates in Separator>

The resin aggregates in the separator were defined in a region with anarea of 100 μm length x≤100 μm width, and with no light permeation, whenseparators obtained by the membrane formation steps in the Examples andComparative Examples described below were observed with a transmissionoptical microscope. The number of resin aggregates per 1000 m² area ofthe separator were counted during observation with a transmissionoptical microscope.

<Transition Temperature for Storage Modulus and Loss Modulus (Version1)>

Using a dynamic viscoelasticity measurement apparatus for dynamicviscoelasticity measurement of the separator, it is possible tocalculate the storage modulus (E′), the loss modulus (E″) and thetransition temperature for the rubber plateau and crystal melt flowregion. The storage modulus change ratio (R_(ΔE′)) was calculated by thefollowing formula (1), the mixed storage modulus ratio (R_(E′mix)) wascalculated by the following formula (2), the loss modulus change ratio(R_(ΔE″)) was calculated by the following formula (3) and the mixed lossmodulus ratio (R_(E″mix)) was calculated by the following formula (4).The measuring conditions were the following (i) to (iv).

(i) The dynamic viscoelasticity measurement was carried out under thefollowing conditions:

-   -   Atmosphere: nitrogen    -   Measuring apparatus: RSA-G2 (TA Instruments)    -   Sample thickness: from 5 μm to 50 μm    -   Measuring temperature range: −50 to 225° C.    -   Temperature-elevating rate: 10° C./min    -   Measuring frequency: 1 Hz    -   Transform mode: sine wave tension mode (linear tension)    -   Initial static tensile load: 0.5 N    -   Initial gap distance (at 25° C.): 25 mm    -   Auto strain adjustment: Enabled (range: 0.05 to 25% amplitude,        0.02 to 5 N sine wave load).

(ii) The static tensile load is the median value of the maximum stressand minimum stress for each periodic motion, and the sine wave load isthe vibrational stress centered on the static tensile load.

(iii) Sine wave tension mode was measurement of the vibrational stresswhile carrying out periodic motion at a fixed amplitude of 0.2%, duringwhich time the vibrational stress was measured while varying the gapdistance and static tensile load so that the difference between thestatic tensile load and the sine wave load was within 20%. When the sinewave load was 0.02 N or lower, the vibrational stress was measured whileamplifying the amplitude value so that the sine wave load was no greaterthan 5 N and the increase in the amplitude value was no greater than25%.

(iv) The storage modulus and loss modulus were calculated from therelationship between the obtained sine wave load and amplitude value,and the following formulas:σ*=σ₀·Exp[i(ωt+δ)],ε*=ε₀·Exp(iωt),σ*=E*·ε*E*=E′+iE″{where σ*: vibrational stress, ε*: strain, i: imaginary number unit, ω:angular frequency, t: time, δ: phase difference between vibrationalstress and strain, E*: complex modulus, E′: storage modulus, E″: lossmodulus,

vibrational stress: sine wave load/initial cross-sectional area

static tensile load: load at minimum point of vibrational stress foreach period (minimum point of gap distance for each period)

sine wave load: difference between measured vibrational stress andstatic tensile load}.

E′_(S), E′_(j) and E″_(S), E″_(j) were the average values in the dynamicviscoelasticity measurement data for each storage modulus or lossmodulus at 160° C. to 220° C. E′_(a), E′₀ and E″_(a), E″₀ were theaverage values in the dynamic viscoelasticity measurement data for eachstorage modulus or loss modulus at 160° C. to 220° C.R _(ΔE′) =E′ _(S) /E′ _(j)  (1) Comparison before and after loading intocellR _(E′mix) =E′ _(a) /E′ ₀  (2) Comparison with and without silanecrosslinkingR _(ΔE″) =E″ _(S) /E″ _(j)  (3) Comparison before and after loading intocellR _(E″mix) =E″ _(a) /E″ ₀  (4) Comparison with and without silanecrosslinking

FIG. 1 shows an example of a graph for illustration of the relationshipbetween temperature and storage modulus. The storage modulus for areference membrane (a separator for an electricity storage device notcontaining a silane-modified polyolefin) and for a crosslinked membranewithin the temperature range of −50° C. to 225° C. were compared asshown in FIG. 1 , and the transition temperature for the rubber plateauand crystal melt flow region of each can be seen in FIG. 1 .Incidentally, the transition temperature is the temperature at theintersection between a straight line extending the baseline from thehigh-temperature end toward the low-temperature end, and the tangentline drawn at the point of inflection of the curve at the section ofcrystal melt transition.

FIG. 2 shows an example of a graph for illustration of the relationshipbetween temperature and loss modulus. In FIG. 2 , the loss modulus for areference membrane (a separator for an electricity storage device notcontaining a silane-modified polyolefin) and for a crosslinked membranewithin the temperature range of −50° C. to 220° C. are compared, and thetransition temperature is shown as determined by the same method as FIG.1 . In the relevant technical field, the storage modulus and lossmodulus are interchangeable as represented by the following formula:tan δ=E″/E′{where tan δ represents the loss tangent, E′ represents the storagemodulus and E″ represents the loss modulus}.

For measurement of the mixed storage modulus ratio (R_(E′mix)) or mixedloss modulus ratio (R_(E″mix)), a non-silane-modified polyolefinmicroporous membrane with a gelation degree of about 0% was used, as aseparator for an electricity storage device not containing asilane-modified polyolefin. When no sample rupture (abrupt reduction inelastic modulus) was observed at 160° C. to 220° C., E′_(a), E′₀, E″_(a)and E″₀ were calculated from the mean value for 160° C. to 220° C., andwhen sample rupture was observed at 160° C. to 220° C., they werecalculated from the mean value from 160° C. to the rupture pointtemperature. For example, the reference membranes shown in FIGS. 1 and 2exhibited rupture at 207° C.

<Transition Temperature for Storage Modulus and Loss Modulus (Version2)>

Using a dynamic viscoelasticity measurement apparatus for dynamicviscoelasticity measurement of the separator, it is possible tocalculate the storage modulus (E′), the loss modulus (E″) and thetransition temperature for the rubber plateau and crystal melt flowregion. The storage modulus change ratio (R_(ΔE′X)) was calculated bythe following formula (1), the mixed storage modulus ratio (R_(E′mix))was calculated by the following formula (2), the mixed loss modulusratio (R_(E″x)) was calculated by the following formula (3) and themixed loss modulus ratio (R_(E″mix)) was calculated by the followingformula (4). As the measuring conditions for measurement of the storagemodulus and loss modulus, an RSA-G2 dynamic viscoelasticity measurementapparatus by TA Instruments was used, with a measuring frequency of 1 Hzand a strain of 0.2%, under a nitrogen atmosphere and in a temperaturerange of −50° C. to 310° C., while the other conditions were accordingto version 1 described above. E′z, E′_(Z0) and E″_(Z), E″^(Z0) were theaverage values in the dynamic viscoelasticity measurement data for eachstorage modulus or loss modulus at 160° C. to 300° C. E′, E′₀ and E″,E″₀ were the average values in the dynamic viscoelasticity measurementdata for each storage modulus or loss modulus at 160° C. to 300° C.R _(ΔE′X) =E′ _(Z) /E′ _(z0)  (1) Comparison before and after loadinginto cellR _(E′mix) =E′/E′ ₀  (2) Comparison with and without amorphouscrosslinked structureR _(E″X) =E″ _(Z) /E″ _(Z0)  (3) Comparison before and after loadinginto cellR _(E″mix) =E″/E″ ₀  (4) Comparison with and without amorphouscrosslinked structure

FIG. 9 shows an example of a graph for illustration of the relationshipbetween temperature and storage modulus. The storage modulus for areference membrane (a separator for an electricity storage devicewithout an amorphous crosslinked structure) and for a crosslinkedmembrane within the temperature range of −50° C. to 310° C. werecompared as shown in FIG. 9 , and the transition temperature for therubber plateau and crystal melt flow region of each can be seen in FIG.9 . Incidentally, the transition temperature is the temperature at theintersection between a straight line extending the baseline from thehigh-temperature end toward the low-temperature end, and the tangentline drawn at the point of inflection of the curve at the section ofcrystal melt transition.

FIG. 10 shows an example of a graph for illustration of the relationshipbetween temperature and loss modulus. In FIG. 10 , the loss modulus fora reference membrane (a separator for an electricity storage device notcontaining a silane-modified polyolefin) and for a crosslinked membranewithin the temperature range of −50° C. to 310° C. are compared, and thetransition temperature is shown as determined by the same method as FIG.9 . In the relevant technical field, the storage modulus and lossmodulus are interchangeable as represented by the following formula:tan δ=E″/E′{where tan δ represents the loss tangent, E′ represents the storagemodulus and E″ represents the loss modulus}.

For measurement of the mixed storage modulus ratio (R_(E′mix)) or mixedloss modulus ratio (R_(E″mix)), a polyolefin microporous membrane with agelation degree of about 0% was used, as a separator for an electricitystorage device without an amorphous crosslinked structure. When nosample rupture (abrupt reduction in elastic modulus) was observed at160° C. to 300° C., E′, E′₀, E″ and E″₀ were calculated from the meanvalue for 160° C. to 300° C., and when sample rupture was observed at160° C. to 300° C., they were calculated from the mean value from 160°C. to the rupture point temperature. For example, the referencemembranes shown in Table 11 and Table 12 and in FIG. 9 and FIG. 10exhibited rupture at 210° C.

For the purpose of the present specification, the separator for anelectricity storage device without an amorphous crosslinked structuremay be a separator produced using any type selected from the groupconsisting of polyethylene: X (viscosity-average molecular weight:100,000 to 400,000), PE: Y (viscosity-average molecular weight: 400,000to 800,000) and PE: Z (viscosity-average molecular weight: 800,000 to9,000,000), or two, three or more types selected from the groupconsisting of X, Y and Z, in admixture in any proportion. A polyolefincomposed entirely of the hydrocarbon backbone of low densitypolyethylene: LDPE, linear low-density polyethylene: LLDPE,polypropylene: PP or an olefin-based thermoplastic elastomer may also beadded to the mixed composition. More specifically, the separator for anelectricity storage device without an amorphous crosslinked structuremay be a polyolefin microporous membrane having a solid content ratechange (hereunder referred to as “gelation degree”) of no greater than10% before and after heating in a decalin solution at 160° C. Duringmeasurement of the gelation degree, the solid content is the resinportion alone, containing no other materials such as inorganicsubstances.

On the other hand, the gelation degree of a polyolefin microporousmembrane with an amorphous crosslinked structure, such as a silanecrosslinking structure, is preferably 30% or greater and more preferably70% or greater.

<Membrane Softening Transition Temperature and Membrane RuptureTemperature for Storage Modulus and Loss Modulus (Version 3)>

Using a dynamic viscoelasticity measurement apparatus for solidviscoelasticity measurement of the separator, it is possible tocalculate the storage modulus (E′), the loss modulus (E″) and themembrane softening transition temperature. The conditions for the solidviscoelasticity measurement were the following (i) to (iv).

(i) The dynamic viscoelasticity measurement is carried out under thefollowing conditions:

-   -   Measuring apparatus: RSA-G2 (TA Instruments)    -   Sample thickness: 200 μm to 400 μm (when the membrane thickness        of the sample alone was less than 200 μm, the dynamic        viscoelasticity measurement was carried out with multiple        samples stacked to a total thickness in the range of 200 μm to        400 μm)    -   Measuring temperature range: −50° C. to 250° C.    -   Temperature-elevating rate: 10° C./min    -   Measuring frequency: 1 Hz    -   Transform mode: sine wave tension mode (linear tension)    -   Initial static tensile load: 0.2 N    -   Initial gap distance (at 25° C.): 10 mm    -   Auto strain adjustment: Disabled.

(ii) The static tensile load is the median value of the maximum stressand minimum stress for each periodic motion, and the sine wave load isthe vibrational stress centered on the static tensile load.

(iii) Sine wave tension mode is measurement of the vibrational stresswhile carrying out periodic motion at a fixed amplitude of 0.1%, whereinin sine wave tension mode, the vibrational stress is measured whilevarying the gap distance and static tensile load so that the differencebetween the static tensile load and the sine wave load is within 5%, andwhen the sine wave load is 0.1 N or lower, the vibrational stress ismeasured with the static tensile load fixed at 0.1 N.

(iv) The storage modulus (E′) and loss modulus (E″) are calculated fromthe relationship between the obtained sine wave load and amplitudevalue, and the following formulas:σ*=σ₀·Exp[i(ωt+δ)],ε*=ε₀·Exp(iωt),σ*=E*·ε*E*=E′+iE″{where σ*: vibrational stress, ε*: strain, i: imaginary number unit, ω:angular frequency, t: time, δ*: phase difference between vibrationalstress and strain, ε*: complex modulus, E′: storage modulus, E″: lossmodulus,

vibrational stress: sine wave load/initial cross-sectional area

static tensile load: load at minimum point of vibrational stress foreach period (minimum point of gap distance for each period)

sine wave load: difference between measured vibrational stress andstatic tensile load}. The average value of the maximum and minimum of E′is calculated as the average E′ (E′_(ave)), and the mean value of themaximum and minimum of E″ is calculated as the average E″ (E″_(ave)).

Incidentally, for E′ and E″, the maximum and minimum were calculated foreach storage modulus or loss modulus at −50° C. to 250° C. among thedynamic viscoelasticity measurement data. More specifically, when nosample rupture (abrupt reduction in elastic modulus) was observed at−50° C. to 250° C., the maximum and minimum at −50° C. to 250° C. werecalculated, and the value at the temperature where sample rupture wasobserved at −50° C. to 250° C. was recorded as the minimum. In therelevant technical field, the storage modulus and loss modulus areinterchangeable as represented by the following formula:tan δ=E″/E′{where tan δ represents the loss tangent, E′ represents the storagemodulus and E″ represents the loss modulus}.

The membrane softening transition temperature is the minimum temperatureamong the dynamic viscoelasticity measurement data at which the curvefor the gap distance of the sample is obtained as a first derivative.The membrane rupture temperature is the temperature at which samplerupture (abrupt reduction in elastic modulus) is observed, among thedynamic viscoelasticity measurement data, with the measurement limittemperature sometimes being established at 250° C. from the viewpoint ofprogression of the thermal decomposition reaction of the polyolefinresin. However, since the same phenomenon can be understood even withmeasurement at temperatures above 250° C., a separator for anelectricity storage device with a membrane rupture temperature of 180°C. or higher can be produced for this embodiment.

<Membrane Rupture Temperature of Layer A>

Using a TMA50™ by Shimadzu Corp. in fixed-length mode, the environmentaltemperature was varied from 25 to 250° C. and the temperature at themoment that the load was fully released was established as the TMAmembrane rupture temperature (the membrane rupture temperature of layerA, measured by TMA).

Specifically, a sample was taken from layer A at 3 mm in the TD and 14mm in the MD, for use as a sample strip (a sample strip with the longside in the MD). Both ends of the sample strip in the MD were set on adedicated probe with the chuck distance at 10 mm, and a load of 1.0 gwas applied to the sample strip. The furnace in which the test piece hadbeen mounted was increased in temperature, and the membrane rupturetemperature (° C.) was recorded as the temperature at which the load wasshown to be 0 g.

When measuring a sample strip TD with the long side in the TD, layer Ais sampled to 14 mm in the TD and 3 mm in the MD and used as the samplestrip, both ends in the TD of the sample are anchored with a chuck to adedicated probe, the chuck distance is set to 10 mm, an initial load of1.0 g is applied, and the same procedure as above is carried out.

<Heat Shrinkage Factor at 150° C.>

The laminated stack before formation of the crosslinked structure (thelaminated stack comprising layer A and layer B) was sampled at 100 mm inthe TD and 100 mm in the MD, for use as a sample strip. The sample stripwas allowed to stand for 1 hour in an oven at 150° C. During this time,the sample strip was sandwiched between two sheets so that the warm airdid not directly contact with the sample strip. After removing thesample strip from the oven and cooling it, the area of the sample stripwas measured, and the heat shrinkage factor at 150° C. (T1) beforeformation of the crosslinked structure was calculated by the followingformula.Heat shrinkage factor at 150° C. (%)=(10,000 (mm²)−area of sample stripafter heating (mm²))×100/10,000

The laminated stack after formation of the crosslinked structure wasalso sampled to 100 mm in the TD and 100 mm in the MD to obtain a samplestrip, and the same procedure as above was carried out, to calculate theheat shrinkage factor at 150° C. (T2) after formation of the crosslinkedstructure.

The heat shrinkage factor (T2) was divided by the heat shrinkage factor(T1) to obtain the ratio (T2/T1). The value of the ratio (T2/T1)corresponds to the change in the heat shrinkage factor at 150° C. (T2)after formation of the crosslinked structure with respect to the heatshrinkage factor at 150° C. (T1) before formation of the crosslinkedstructure.

<Battery Destruction Safety Test 1>

Battery destruction safety test 1 is a test in which a battery chargedto 4.5 V is hit with an iron nail at a speed of 20 mm/sec and puncturedto produce internal short circuiting. This test can measuretime-dependent change behavior of voltage reduction of the battery dueto internal short circuiting, and battery surface temperature increasebehavior due to internal short circuiting, to elucidate these phenomenaduring internal short circuiting. Inadequate shutdown function of theseparator during internal short circuiting or membrane rupture at lowtemperature can also result in sharp heat release of the battery, whichmay lead to ignition of the electrolyte solution and fuming and/orexplosion of the battery.

(Fabrication of Battery to be Used in Battery Destruction Safety Test 1)

1a. Fabrication of Positive Electrode

A slurry was prepared by sampling 92.2 weight % of lithium cobaltcomposite oxide (LiCoO₂) as a positive electrode active material, 2.3weight % each of flaky graphite and acetylene black as conductivematerials and 3.2 weight % of polyvinylidene fluoride (PVDF) as a resinbinder, and dispersing them in N-methylpyrrolidone (NMP). The slurry wascoated using a die coater onto one side of a 20 μm-thick aluminum foilas the positive electrode collector, and dried at 130° C. for 3 minutes,after which it was compression molded using a roll press. During thistime, the active material coating amount on the positive electrode wasadjusted to 250 g/m² and the active material bulk density was adjustedto 3.00 g/cm³.

1b. Fabrication of Negative Electrode

A slurry was prepared by dispersing 96.9 weight % of artificial graphiteas a negative electrode active material, 1.4 weight % of carboxymethylcellulose ammonium salt as a resin binder and 1.7 weight % ofstyrene-butadiene copolymer latex in purified water. The slurry wascoated using a die coater onto one side of a 12 μm-thick copper foil asthe negative electrode collector, and dried at 120° C. for 3 minutes,after which it was compression molded using a roll press. During thistime, the active material coating amount on the negative electrode wasadjusted to 106 g/m² and the active material bulk density was adjustedto 1.35 g/cm³.

1c. Preparation of Nonaqueous Electrolyte Solution

A 1.0 mol/L portion of concentrated LiPF₆, as a solute, was dissolved ina mixed solvent of ethylene carbonate:ethylmethyl carbonate=1:2 (volumeratio), to prepare a nonaqueous electrolyte solution.

1d. Battery Assembly

A separator was cut out to 60 mm in the widthwise (TD) direction and1000 mm in the lengthwise (MD) direction, the separator was folded in ahairpin fashion, and positive electrodes and negative electrodes werealternately stacked between the separator (12 positive electrodes, 13negative electrodes). The positive electrodes used had areas of 30 mm×50mm, and the negative electrodes had areas of 32 mm×52 mm. The laminatedstack that had been folded in a hairpin fashion was inserted into alaminating bag, and then injected with the nonaqueous electrolytesolution obtained in c. above and sealed. After allowing it to stand atroom temperature for 1 day, it was subjected to initial charge of thefabricated battery for a total of 6 hours, by a method of charging to acell voltage of 4.2 V at a current value of 3 mA (0.5 C) in anatmosphere of 25° C. and, after reaching that voltage, beginning to drawout a current of 3 mA while maintaining 4.2 V. The battery was thendischarged to a cell voltage of 3.0 V at a current value of 3 mA (0.5C).

(Maximum Heat Release Rate)

After puncturing the obtained battery with an iron nail, the batterysurface temperature was measured using a thermocouple for a period of300 seconds and the resulting temperature change graph was used todetermine the rate during which the change in temperature increase persecond was greatest, as the maximum heat release rate.

(Voltage Reduction Time)

The time required for voltage reduction from 4.5 V to 3 V afterpuncturing the obtained battery with an iron nail was established as thevoltage reduction time (3 V reduction time).

<Cycle Characteristic Evaluation and Battery Fabrication Method>

A battery for evaluation of cycle characteristics was fabricated by thesame method as in 1a. to 1c. above for the method of fabricating abattery used in <Battery destruction safety test 1>, but with theassembly described in 1d-2. below.

1d-2. Battery Assembly

The separator was cut out to a circle with a diameter of 18 mm and thepositive electrode and negative electrode to circles with diameters of16 mm, and the positive electrode, separator and negative electrode werestacked in that order with the active material sides of the positiveelectrode and negative electrode facing each other, after which theywere housed in a covered stainless steel metal container. The containerand cover were insulated, with the container in contact with thenegative electrode copper foil and the cover in contact with thepositive electrode aluminum foil. The nonaqueous electrolyte solutionobtained in 1c. under <Battery destruction safety test 1> above wasinjected into the container, which was then sealed. After allowing it tostand at room temperature for 1 day, it was subjected to initial chargeof the fabricated battery for a total of 6 hours, by a method ofcharging to a cell voltage of 4.2 V at a current value of 3 mA (0.5 C)in an atmosphere of 25° C. and, after reaching that voltage, beginningto draw out a current of 3 mA while maintaining 4.2 V. The battery wasthen discharged to a cell voltage of 3.0 V at a current value of 3 mA(0.5 C).

Charge-discharge of the obtained battery was carried out for 100 cyclesin an atmosphere of 60° C. Charging was for a total of 3 hours, by amethod of charging to a cell voltage of 4.2 V at a current value of 6.0mA (1.0 C) and, after reaching that voltage, beginning to draw out acurrent of 6.0 mA while maintaining 4.2 V. Discharge was to a cellvoltage of 3.0 V at a current value of 6.0 mA (1.0 C).

(Cycle Characteristic Evaluation 1)

The capacity retention was calculated from the service capacity at the100th cycle and the service capacity at the first cycle. A high capacityretention was evaluated as a satisfactory cycle characteristic.

(Cycle Characteristic Evaluation 2)

The capacity retention (%) was calculated from the service capacity atthe 300th cycle and the service capacity at the first cycle, based onthe following formula. A high capacity retention was evaluated as asatisfactory cycle characteristic.Evaluation result (%)=(100×retention volume after 300 cycles/servicecapacity at first cycle)<Fuse/Meltdown (F/MD) Characteristic>(i) Pressure of 0.5 MPa and Temperature-Elevating Rate of 2° C./Min

A circular positive electrode, separator and negative electrode withdiameters of 200 mm were cut out and stacked, and a nonaqueouselectrolyte solution was added to the obtained laminated stack andallowed to thoroughly permeate it. The laminated stack is insertedbetween the center section of a circular aluminum heater with a diameterof 600 mm, and the aluminum heater is pressed vertically with ahydraulic jack to 0.5 MPa, thus completing preparation for themeasurement. The laminated stack is heated with the aluminum heater at atemperature-elevating rate of 2° C./min while measuring the resistance(Ω) between the electrodes. Resistance between the electrodes increaseswith fusing of the separator, and the temperature when the resistancefirst exceeds 1000Ω is recorded as the fuse temperature (shutdowntemperature). Heating is continued, and the temperature when theresistance falls below 1000Ω is recorded as the meltdown temperature(membrane rupture temperature).

(ii) Maximum Pressurization of 10 MPa and Temperature-Elevating Rate of15° C./Min

A circular positive electrode, separator and negative electrode withdiameters of 200 mm were cut out and stacked, and a nonaqueouselectrolyte solution was added to the obtained laminated stack andallowed to thoroughly permeate it. The laminated stack was insertedbetween the center section of a circular aluminum heater with a diameterof 600 mm, and the aluminum heater was pressed vertically with ahydraulic jack to a pressure of 10 MPa, thus completing preparation forthe measurement. The laminated stack was heated with the aluminum heaterat a temperature-elevating rate of 15° C./min while measuring theresistance (Ω) between the electrodes. Resistance between the electrodesincreased, and the temperature when the resistance first exceeded 1000Ωwas recorded as the shutdown temperature (° C.). Heating was furthercontinued, and the temperature when the resistance fell below 1000Ω wasrecorded as the meltdown temperature (° C.).

For the evaluations of both (i) and (ii), a resistance measurement wirewas bonded with conductive silver paste behind the aluminum foil of thepositive electrode fabricated according to “1a. Fabrication of positiveelectrode” under <Battery destruction safety test 1> above. In addition,a resistance measurement wire was bonded with conductive silver pastebehind the negative electrode copper foil fabricated according to “1b.Fabrication of negative electrode” under <Battery destruction safetytest 1> above. An electrolyte-containing solution prepared according to“1c. Preparation of nonaqueous electrolyte solution” under <Batterydestruction safety test 1> above was also used for the F/MD propertytest.

<Safety Test (Nail Penetration Test) 2>

2a. Fabrication of Positive Electrode

After mixing 90.4 weight % of a nickel, manganese and cobalt compositeoxide (NMC) (Ni:Mn:Co=1:1:1 (element ratio), density: 4.70 g/cm³), asthe positive electrode active material, 1.6 weight % of graphite powder(KS6) (density: 2.26 g/cm³, number-mean particle size: 6.5 μm) and 3.8weight % of acetylene black powder (AB) (density: 1.95 g/cm³,number-mean particle size: 48 nm), as conductive aids, and 4.2 weight %of PVDF (density: 1.75 g/cm³) as a resin binder, the mixture wasdispersed in NMP to prepare a slurry. The slurry was coated using a diecoater onto one side of a 20 μm-thick aluminum foil sheet as thepositive electrode collector, and dried at 130° C. for 3 minutes, afterwhich it was compression molded using a roll press, to fabricate apositive electrode. The coating amount of the positive electrode activematerial was 109 g/m².

2b. Fabrication of Negative Electrode

In purified water there were dispersed 87.6 weight % of graphite powderA (density: 2.23 g/cm³, number-mean particle size: 12.7 μm) and 9.7weight % of graphite powder B (density: 2.27 g/cm³, number-mean particlesize: 6.5 μm) as negative electrode active materials, and 1.4 (solid)weight % of carboxymethyl cellulose ammonium salt (1.83 weight % solidconcentration aqueous solution) and 1.7 (solid) weight % of diene rubberlatex (40 weight % solid concentration aqueous solution) as resinbinders, to prepare a slurry. The slurry was coated using a die coateronto one side of a 12 μm-thick copper foil sheet as the negativeelectrode collector, and dried at 120° C. for 3 minutes, after which itwas compression molded using a roll press to fabricate a negativeelectrode. The coating amount of the negative electrode active materialwas 52 g/m².

2c. Preparation of Nonaqueous Electrolyte Solution

A 1.0 mol/L portion of concentrated LiPF₆, as a solute, was dissolved ina mixed solvent of ethylene carbonate:ethyl methyl carbonate=1:2 (volumeratio), to prepare a nonaqueous electrolyte solution.

2d. Fabrication of Battery

The positive electrode, negative electrode and nonaqueous electrolytesolution obtained in 2a to 2c above, and a separator (a separator of theExamples or a separator of the Comparative Examples) were used tofabricate a laminated secondary battery with a size of 100 mm×60 mm anda capacity of 3 Ah, which was charged with constant current, constantvoltage (CCCV) over a period of 3 hours under conditions with a currentvalue of 1 A (0.3 C) and a final cell voltage of 4.2 V.

2e. Nail Penetration Evaluation

The fabricated laminated secondary battery was set on a steel sheet in atemperature-adjustable explosion-proof booth. Setting theexplosion-proof booth interior to a temperature of 40° C., the centersection of the laminated secondary battery was punctured with an ironnail having a diameter of 3.0 mm at a speed of 2 mm/sec, and the nailwas left in the punctured state. A thermocouple had been set inside thenail so as to allow measurement inside the laminated battery afterpuncturing with the nail, and its temperature was measured and thepresence or absence of ignition was evaluated.

The evaluation was repeated using laminated secondary batteries newlyfabricated by the same method, and the number of samples withoutignition (no ignition) was calculated as a percentage value by thefollowing formula.Evaluation result (%)=(100×number of samples without ignition/totalnumber of samples)

The passing rate in the nail penetration evaluation is preferably 50% orgreater with 200 cycles and 5% or greater with 1000 cycles.

<Experiment Group I>

[Silane Graft-Modified Polyolefin Production Method]

The polyolefin starting material to be used as the silane graft-modifiedpolyolefin may be one with a viscosity-average molecular weight (Mv) of100,000 to 1,000,000, a weight-average molecular weight (Mw) of 30,000to 920,000, and a number-average molecular weight of 10,000 to 150,000,and it may be propylene or a butene-copolymerized α-olefin. After meltkneading the polyethylene starting material with an extruder whileadding an organic peroxide (di-t-butyl peroxide) and generating radicalsin the polymer chain of the α-olefin, it is filled withtrimethoxyalkoxide-substituted vinylsilane and addition reaction iscarried out to introduce alkoxysilyl groups into the α-olefin polymer,forming a silane-graft structure. A suitable amount of an antioxidant(pentaerythritoltetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate])is simultaneously added to adjust the radical concentration in thesystem, thus inhibiting chain-style chain reaction (gelation) in theα-olefin. The obtained silane-grafted polyolefin molten resin is cooledin water and pelletized, after which it is heat-dried at 80° C. for 2days and the water and unreacted trimethoxyalkoxide-substitutedvinylsilane are removed. The residual concentration of the unreactedtrimethoxyalkoxide-substituted vinylsilane in the pellets is about 10 to1500 ppm.

The silane graft-modified polyolefin obtained by this method is used asthe “Silane-modified polyethylene (B)” in Table 8.

Example I-1

To 79.2 weight % of polyethylene homopolymer (A) with a weight-averagemolecular weight of 500,000 there was added 19.8 weight % ofsilane-grafted polyethylene (silane-modified polyethylene (B)) with anMFR (190° C.) of 0.4 g/min, obtained using a polyolefin with aviscosity-average molecular weight of 20,000 as starting material andmodification reaction with trimethoxyalkoxide-substituted vinylsilane(the respective contents of resin compositions (A) and (B) thus being0.8 and 0.2), and 1 weight % ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]as an antioxidant, and these were dry blended using a tumbler blender toobtain a mixture. The obtained mixture was supplied to a twin-screwextruder through a feeder, under a nitrogen atmosphere. Also, liquidparaffin (kinematic viscosity at 37.78° C.: 7.59×10⁻⁵ m²/s) was injectedinto the extruder cylinder by a plunger pump.

The mixture was melt kneaded with liquid paraffin in an extruder, andadjusted with a feeder and pump so that the quantity ratio of liquidparaffin in the extruded polyolefin composition was 70 weight % (i.e. apolymer concentration of 30 weight %). The melt kneading conditions werea preset temperature of 220° C., a screw rotational speed of 240 rpm anda discharge throughput of 18 kg/h.

The melt kneaded mixture was then extrusion cast through a T-die onto acooling roll controlled to a surface temperature of 25° C., to obtain agel sheet (molded sheet) with a raw membrane thickness of 1400 μm.

The molded sheet was then simultaneously fed into a biaxial tenterstretching machine for biaxial stretching, to obtain a stretched sheet.The stretching conditions were an MD factor of 7.0, a TD factor of 6.0(i.e. a factor of 7×6), and a biaxial stretching temperature of 125° C.

The stretched gel sheet was subsequently fed into a methyl ethyl ketonetank and thoroughly immersed in the methyl ethyl ketone for extractionremoval of the liquid paraffin, after which the methyl ethyl ketone wasdried off to obtain a porous body.

The porous body to be subjected to heat setting (HS) was fed to a TDtenter and HS was carried out at a heat setting temperature of 125° C.and a stretch ratio of 1.8, after which relaxation was carried out to afactor of 0.5 in the TD direction (i.e. the HS relaxation factor was0.5), to obtain a microporous membrane.

The obtained microporous membrane was then cut at the edges and wound upas a mother roll with a width of 1,100 mm and a length of 5,000 m.

During the evaluation, the microporous membrane wound out from themother roll was slit as necessary for use as the evaluation separator A.

Examples I-2 to I-6

The microporous membranes listed in Table 8 were obtained by the sameprocedure as Example I-1, except for changing the quantity ratio ofcomponents A and B and the crosslinking method and conditions as shownin Table 8.

Comparative Examples I-1 and I-2

To 79.2 weight % of polyethylene homopolymer (A) with a weight-averagemolecular weight of 500,000 there was added 19.8 weight % ofsilane-grafted polyethylene (silane-modified polyethylene (B)) with anMFR (190° C.) of 0.4 g/min, obtained using a polyolefin with aviscosity-average molecular weight of 20,000 as starting material andmodification reaction with trimethoxyalkoxide-substituted vinylsilane(the respective contents of resin compositions (A) and (B) thus being0.8 and 0.2), and 1 weight % ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]as an antioxidant, and these were dry blended using a tumbler blender toobtain a mixture. The obtained mixture was supplied to a twin-screwextruder through a feeder, under a nitrogen atmosphere. Also, liquidparaffin (kinematic viscosity at 37.78° C.: 7.59×10⁻⁵ m²/s) was injectedinto the extruder cylinder by a plunger pump.

The mixture was melt kneaded with liquid paraffin in an extruder, andadjusted with a feeder and pump so that the quantity ratio of liquidparaffin in the extruded polyolefin composition was 70 weight % (i.e. apolymer concentration of 30 weight %). The melt kneading conditions werea preset temperature of 220° C., a screw rotational speed of 240 rpm anda discharge throughput of 18 kg/h.

The melt kneaded mixture was then extrusion cast through a T-die onto acooling roll controlled to a surface temperature of 25° C., to obtain agel sheet (molded sheet) with a raw membrane thickness of 1400 μm.

The molded sheet was then simultaneously fed into a biaxial tenterstretching machine for biaxial stretching, to obtain a stretched sheet.The stretching conditions were an MD factor of 7.0, a TD factor of 6.0(i.e. a factor of 7×6), and a biaxial stretching temperature of 125° C.

The stretched gel sheet was subsequently fed into a methyl ethyl ketonetank and thoroughly immersed in the methyl ethyl ketone for extractionremoval of the liquid paraffin, after which the methyl ethyl ketone wasdried off to obtain a porous body.

The porous body to be subjected to heat setting (HS) was fed to a TDtenter and HS was carried out at a heat setting temperature of 125° C.and a stretch ratio of 1.8, after which relaxation was carried out to afactor of 0.5 in the TD direction (i.e. the HS relaxation factor was0.5).

It was also fed into an ethanol bath (affinity treatment tank) andimmersed and retained for 60 seconds for affinity treatment of theheat-treated porous body to obtain an affinity-treated porous body.

Each affinity-treated porous body was also fed, immersed and retainedfor 60 seconds in a 25% aqueous caustic soda solution (temperature: 80°C., pH 8.5 to 14) for Comparative Example I-1 and in an aqueous 10%hydrochloric acid solution (temperature: 60° C., pH 1 to 6.5) forComparative Example I-2, for crosslinking treatment of theaffinity-treated porous body, to obtain a crosslinked porous body.

The crosslinked porous body was fed into water (washing treatment tank)and immersed and retained for 60 seconds for washing of the crosslinkedporous body. It was then fed to a conveyor dryer and dried at 120° C.for 60 seconds to obtain a microporous membrane.

The obtained microporous membrane was then cut at the edges and wound upas a mother roll with a width of 1,100 mm and a length of 5,000 m.

During the evaluation, the microporous membrane wound out from themother roll was slit as necessary for use as the evaluation separator A.

[Evaluation Results]

The microporous membranes and batteries obtained in Examples I-1 to I-6and Comparative Examples I-1 and I-2 were evaluated by each of theevaluation methods described above, and the evaluation results are shownin Table 8. FIG. 3 shows the relationship between temperature andresistance for a battery comprising the microporous membrane obtained inExample I-1 as the separator. FIG. 3 and Table 8 show that the shutdowntemperature of the separator obtained in Example I-1 is 143° C. and themembrane rupture temperature is ≥200° C. FIG. 13 shows an ¹H and ¹³C-NMRchart (b) for the separator obtained in Example I-1, in the state beforecrosslinking.

TABLE 8 Example I 1 2 3 4 Microporous Resin Polyethylene(A) Weight 0.80.94 0.62 0.96 membrane composition ratio Silane-modified Weight 0.20.06 0.38 0.04 polyethylene(B) ratio Kneading temperature ° C. 220 220220 220 Crosslinking Method — — — — method Crosslinking reaction timingReagent Temperature ° C. pH Basic Membrane thickness um 11 11 11 11separator Porosity (i) % 40 39 40 38 properties Air permeability sec/100cm³ 160 170 163 161 Shutdown/rupture Shutdown ° C. 143 142 143 153resistance temperature (i) Membrane ° C. ≥200 ≥200 ≥200 160 rupturetemperature (i) Resin aggregates in separator /1000 m² 2 3 2 1 Storagemodulus change R_(ΔE′) Factor 2.1 1.7 16 1.1 factor, ver. 1 R_(E′mix)Factor 8.5 2.1 15 1.1 Loss modulus change R_(ΔE″) Factor 1.9 1.6 15 1.1factor, ver. 1 R_(E″mix) Factor 6.2 3 13 1.1 Transition temperaturecalculated ° C. 143 140 142 151 by storage modulus ver. 1 BatteryCrosslinking method Contact with Contact with Contact with Contact withelectrolyte electrolyte electrolyte electrolyte solution solutionsolution solution until until until until initial initial initialinitial charge- charge- charge- charge- discharge discharge dischargedischarge Battery cycle stability 1 % 98 97 98 78 Battery Internalmaximum ° C./sec 6 9 8 12 destruction heat release rate safety 1 Voltagereduction sec >300 >300 >300 210 (3 V reduction time) ComparativeExample I Example I 5 6 1 2 Microporous Resin Polyethylene(A) Weight0.58 0 0.8 0.8 membrane composition ratio Silane-modified Weight 0.42 10.2 0.2 polyethylene(B) ratio Kneading temperature ° C. 220 220 220 220Crosslinking Method — — Alkali Acid method treatment treatmentCrosslinking After After reaction pore pore timing formation formationReagent NaOHaq HClaq Temperature ° C. 80 60 pH 8.5 to 14 1 to 6.5 BasicMembrane thickness um 10.5 9 11 11 separator Porosity (i) % 42 39 40 40properties Air permeability sec/100 cm³ 180 195 160 160 Shutdown/ruptureShutdown ° C. 178 179 165 171 resistance temperature (i) Membrane ° C.≥200 ≥200 ≥200 ≥200 rupture temperature (i) Resin aggregates inseparator /1000 m² 562 890 2 2 Storage modulus change R_(ΔE′) Factor 2323 22 25 factor, ver. 1 R_(E′mix) Factor 23 23 21.5 24 Loss moduluschange R_(ΔE″) Factor 22.5 22.5 21 24.5 factor, ver. 1 R_(E″mix) Factor22 22 20.3 22 Transition temperature calculated ° C. 134 133 155 157 bystorage modulus ver. 1 Battery Crosslinking method Contact with Contactwith — — electrolyte electrolyte solution solution until until initialinitial charge- charge- discharge discharge Battery cycle stability 1 %75 70 65 63 Battery Internal maximum ° C./sec 13 15 27 29 destructionheat release rate safety 1 Voltage reduction sec 224 232 3 1 (3 Vreduction time)

The “silane-modified polyethylene (B)” in Table 8 is a silane-modifiedpolyethylene with a density of 0.95 g/cm³ and a melt mass-flow rate(MFR) of 0.4 g/min at 190° C., obtained by modification reaction with atrimethoxyalkoxide-substituted vinylsilane, using a polyolefin with aviscosity-average molecular weight of 20,000 as the starting material.

Experiment Group IIa

[Silane Graft-Modified Polyolefin Production Method]

The polyolefin starting material to be used as the silane graft-modifiedpolyolefin may be one with a viscosity-average molecular weight (Mv) of100,000 to 1,000,000, a weight-average molecular weight (Mw) of 30,000to 920,000, and a number-average molecular weight of 10,000 to 150,000,and it may be ethylene homopolymer, or a copolymerized α-olefin ofethylene and propylene or butene. After melt kneading the polyethylenestarting material with an extruder while adding an organic peroxide(di-t-butyl peroxide) and generating radicals in the polymer chain ofthe α-olefin, it is filled with trimethoxyalkoxide-substitutedvinylsilane and addition reaction is carried out to introducealkoxysilyl groups into the α-olefin polymer, forming a silane-graftstructure. A suitable amount of an antioxidant(pentaerythritoltetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate])is simultaneously added to adjust the radical concentration in thesystem, thus inhibiting chain-style chain reaction (gelation) in theα-olefin. The obtained silane-grafted polyolefin molten resin is cooledin water and pelletized, after which it is heat-dried at 80° C. for 2days and the water and unreacted trimethoxyalkoxide-substitutedvinylsilane are removed. The residual concentration of the unreactedtrimethoxyalkoxide-substituted vinylsilane in the pellets is about 1500ppm or lower.

The silane graft-modified polyethylene obtained by this method is usedas the “Silane-modified polyethylene (B)” in Table 9.

Example II-1

To 80 weight % of polyethylene homopolymer with a weight-averagemolecular weight of 700,000 (polyethylene (A)) there was added 20 weight% of silane-grafted polyethylene (silane-modified polyethylene (B)) withan MFR (190° C.) of 0.4 g/min, obtained using a polyolefin with aviscosity-average molecular weight of 10,000 as starting material andmodification reaction with trimethoxyalkoxide-substituted vinylsilane(the respective contents of resin compositions (A) and (B) thus being80% and 20%), and 1 weight % ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]as an antioxidant, and these were dry blended using a tumbler blender toobtain a mixture. The obtained mixture was supplied to a twin-screwextruder through a feeder, under a nitrogen atmosphere. Also, liquidparaffin (kinematic viscosity at 37.78° C.: 7.59×10⁻⁵ m²/s) was injectedinto the extruder cylinder by a plunger pump.

The mixture was melt kneaded with liquid paraffin in an extruder, andadjusted with a feeder and pump so that the quantity ratio of liquidparaffin in the extruded polyolefin composition was 70 weight % (i.e. apolymer concentration of 30 weight %). The melt kneading conditions werea preset temperature of 220° C., a screw rotational speed of 240 rpm anda discharge throughput of 18 kg/h.

The melt kneaded mixture was then extrusion cast through a T-die onto acooling roll controlled to a surface temperature of 25° C., to obtain agel sheet (molded sheet) with a raw membrane thickness of 1100 μm.

The molded sheet was simultaneously fed into a biaxial tenter stretchingmachine for biaxial stretching, to obtain a stretched sheet. Thestretching conditions were an MD factor of 7.0, a TD factor of 6.2 and abiaxial stretching temperature of 120° C.

The stretched gel sheet was then fed into a dichloromethane tank andthoroughly immersed in the dichloromethane for extraction removal of theliquid paraffin, after which the dichloromethane was dried off to obtaina porous body.

The porous body to be subjected to heat setting (HS) was fed to a TDtenter and HS was carried out at a heat setting temperature of 133° C.and a stretch ratio of 2.1, after which relaxation was carried out to afactor of 2.0 in the TD direction.

The obtained microporous membrane was then cut at the edges and wound upas a mother roll with a width of 1,100 mm and a length of 5,000 m.

During the evaluation, the microporous membrane wound out from themother roll was slit as necessary for use as the evaluation separator A.

Examples II-2 to II-8 and Comparative Examples II-1 to II-3

The microporous membranes listed in Table 9 were obtained by the sameprocedure as Example II-1, except for changing the quantity ratio ofcomponents A and B, the other resin (C) as an additional component, themembrane properties and the crosslinking method and conditions as shownin Table 9. As component “PP” in Table 9 there was usednon-silane-modified polypropylene having an MFR of 2.5 g/10 min or lowerand a density of 0.89 g/cm³ or greater, as measured under conditionswith a temperature of 230° C. and a weight of 2.16 kg. For the “alkalitreatment crosslinking” as the crosslinking method in Table 9, thesample was treated with a 25% aqueous caustic soda solution(temperature: 80° C., pH 8.5 to 14).

[Evaluation Results]

The microporous membranes and batteries obtained in Examples II-1 toII-8 and Comparative Examples II-1 and II-3 were evaluated by each ofthe evaluation methods described above, and the evaluation results areshown in Table 9. Viscoelasticity measurement was carried out using theobtained microporous membrane as the separator for an electricitystorage device, and the relationship between temperature, gap distance,storage modulus and loss modulus is shown in FIG. 4(a) for Example II-1and in FIG. 4(b) for Comparative Example II-1, while the membranesoftening transition temperature determined based on temperature, gapdistance and first derivative of the gap displacement is shown in FIG.5(a) for Example II-1 and FIG. 5(b) for Comparative Example II-1. InExample II-1 to II-8 and Comparative Example II-3, no membrane rupturewas observed at a measurement limit temperature of 250° C. In ExampleII-1 and Comparative Example II-1, the storage modulus, loss modulus,membrane softening transition temperature and membrane rupturetemperature were measured using 26 membranes each with a thickness of 8μm, stacked for a total sample membrane thickness of 208 μm.

TABLE 9A Example II 1 2 3 4 5 6 Microporous Resin Polyethylene(A) weight% 80 20 99 80 80 12 membrane composition Silane-modified weight % 20 801 20 20 83 polyethylene(B) Other resin (C) weight % 0 0 0 0 0 PP/5 BasicMembrane thickness μm 8 11 8 8 12 8 separator Porosity (i) % 40 38 35 3935 41 properties Air permeability sec/100 cm³ 150 156 155 120 122 117Separator Maximum elastic Storage MPa 295 9,500 310 271 7,500 233 solidmodulus, −50 modulus (E′) viscoelasticity to 250° C. Loss MPa 87 3,20075 56 5,300 74 ver. 3 data modulus (E″) Minimum elastic Storage MPa 2.281.80 2.30 1.18 1.77 1.15 modulus, −50 modulus (E′) to 250° C. Loss MPa0.89 0.40 0.70 0.80 0.91 0.53 modulus (E″) Maximum elastic Storage MPa7.17 7.30 35.50 6.80 8.20 32.20 modulus, membrane modulus (E′) softeningtransition Loss MPa 2.16 2.11 11.20 2.03 3.23 12.80 temperature to 250°C. modulus (E″) Minimum elastic Storage MPa 2.28 1.80 2.30 1.18 1.771.15 modulus, membrane modulus (E′) softening transition Loss MPa 0.890.40 0.70 0.80 0.91 0.53 temperature to 250° C. modulus (E″) Membranesoftening transition temperature ° C. 147 149 149 132 155 131 Membranerupture temperature ° C. No No No No No No membrane membrane membranemembrane membrane membrane rupture rupture rupture rupture rupturerupture at 250 at 250 at 250 at 250 at 250 at 250 Resin aggregates inseparator /1000 m² 0 1 0 5 3 8 Battery Crosslinking method ContactContact Contact Contact Contact Contact with with with with with withelectrolyte electrolyte electrolyte electrolyte electrolyte electrolytesolution solution solution solution solution solution until until untiluntil until until initial initial initial initial initial initialcharge- charge- charge- charge- charge- charge- discharge dischargedischarge discharge discharge discharge Battery cycle stability 1 % 9997 98 93 91 81 Passing rate in battery safety test 1 % 95 96 87 83 81 72Comparative Example II Example II 7 8 1 2 3 Microporous ResinPolyethylene(A) weight % 35 80 100 100 2 membrane compositionSilane-modified weight % 40 20 0 0 70 polyethylene(B) Other resin (C)weight % PP/25 0 0 0 PP/28 Basic Membrane thickness μm 12 8 8 8 10separator Porosity (i) % 32 40 39 42 33 properties Air permeabilitysec/100 cm³ 121 150 155 152 175 Separator Maximum elastic Storage MPa7,700 305 1,230 17,500 18,000 solid modulus, −50 modulus (E′)viscoelasticity to 250° C. Loss MPa 4,850 88 338 10,100 10,150 ver. 3data modulus (E″) Minimum elastic Storage MPa 3.32 2.21 0.11 0.10 23.00modulus, −50 modulus (E′) to 250° C. Loss MPa 1.63 1.02 0.07 0.06 12.70modulus (E″) Maximum elastic Storage MPa 27.40 7.30 6.94 6.80 83.00modulus, membrane modulus (E′) softening transition Loss MPa 11.20 2.212.19 2.11 53.50 temperature to 250° C. modulus (E″) Minimum elasticStorage MPa 3.32 2.35 0.11 0.10 23.00 modulus, membrane modulus (E′)softening transition Loss MPa 1.63 0.75 0.07 0.06 12.70 temperature to250° C. modulus (E″) Membrane softening transition temperature ° C. 154146 148 148 155 Membrane rupture temperature ° C. No No 176 178 Nomembrane membrane membrane rupture rupture rupture at 250 at 250 at 250Resin aggregates in separator /1000 m² 7 0 0 330 553 BatteryCrosslinking method Contact Alkali with treatment electrolytecrosslinking solution until initial charge- discharge Battery cyclestability 1 % 72 99 87 42 56 Passing rate in battery safety test 1 % 7593 0 0 38

TABLE 9B Example II 1 2 3 4 5 6 Microporous Separator Membrane Mean MPa4.725 4.550 18.90 3.99 4.985 16.675 membrane solid softening storageviscoelasticity transition modulus ver. 3 data temperature (E′ave) to250° C. Mean MPa 1.525 1.255 5.95 1.415 2.07 6.665 loss modulus (E″ave)Comparative Example II Example II 7 8 1 2 3 Microporous SeparatorMembrane Mean MPa 15.36 4.825 3.525 3.45 53 membrane solid softeningstorage viscoelasticity transition modulus ver. 3 data temperature(E′ave) to 250° C. Mean MPa 6.415 1.48 1.13 1.085 33.1 loss modulus(E″ave)<Experiment Series IIb>[Reference Membrane]

As a separator for an electricity storage device not containing asilane-modified polyolefin (hereunder referred to as “referencemembrane”) there was used a non-silane-graft-modified polyolefinmicroporous membrane having a solid content rate change (hereunderreferred to as “gelation degree”) of about 0%, before and after heatingin a decalin solution at 160° C. During measurement of the gelationdegree, the solid content is the resin portion alone, containing noother materials such as inorganic substances.

For the purpose of the present specification, the separator for anelectricity storage device not containing a silane graft-modifiedpolyolefin may be one produced using any type selected from the groupconsisting of polyethylene (PE): X (viscosity-average molecular weight:100,000 to 400,000), PE: Y (viscosity-average molecular weight: 400,000to 800,000) and PE: Z (viscosity-average molecular weight: 800,000 to9,000,000), or two, three or more types selected from the groupconsisting of X, Y and Z, in admixture in any proportion. A polyolefincomposed entirely of the hydrocarbon backbone of low densitypolyethylene: LDPE, linear low-density polyethylene: LLDPE,polypropylene: PP or an olefin-based thermoplastic elastomer may also beadded to the mixed composition.

[Crosslinked Membrane]

As a separator for an electricity storage device after silanecrosslinking reaction (hereunder referred to as “crosslinked membrane”)there was used the polyolefin microporous membrane of Example II-1 aftercontact with the electrolyte solution as described above, or thepolyolefin microporous membrane of Example II-1 that had been removedfrom the cell after initial charge-discharge and dried. The gelationdegree of the crosslinked membrane was 30% or 70%.

[Viscoelastic Behavior]

A reference membrane and crosslinked membrane were subjected to themeasurement described above under <Membrane softening transitiontemperature and membrane rupture temperature for storage modulus andloss modulus (version 3)>. The measurement results are shown in Table10.

TABLE 10 Viscoelasticity (ver. 3) behavior E′, MPa E″, MPa tanδReference Temperature 147.89 Softening 6.94 2.189 0.32 membrane ° C.176.27 Rupture 0.107 0.068 0.64 Crosslinked Temperature 146.8 Softening7.17 2.16 0.30 membrane ° C. 222.5 Slight reduction in 2.34 0.91 0.39elastic modulus 253.7 No rupture, measurement 2.38 1.04 0.44 completed<Experiment Group III>[Silane Graft-Modified Polyolefin Production Method]

The polyolefin starting material to be used as the silane graft-modifiedpolyolefin may be one with a viscosity-average molecular weight (Mv) of100,000 to 1,000,000, a weight-average molecular weight (Mw) of 30,000to 920,000, and a number-average molecular weight of 10,000 to 150,000,and it may be propylene or a butene-copolymerized α-olefin. After meltkneading the polyethylene starting material with an extruder whileadding an organic peroxide (di-t-butyl peroxide) and generating radicalsin the polymer chain of the α-olefin, it is filled withtrimethoxyalkoxide-substituted vinylsilane and addition reaction iscarried out to introduce alkoxysilyl groups into the α-olefin polymer,forming a silane-graft structure. A suitable amount of an antioxidant(pentaerythritoltetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate])is simultaneously added to adjust the radical concentration in thesystem, thus inhibiting chain-style chain reaction (gelation) in theα-olefin. The obtained silane-grafted polyolefin molten resin is cooledin water and pelletized, after which it is heat-dried at 80° C. for 2days and the water and unreacted trimethoxyalkoxide-substitutedvinylsilane are removed. The residual concentration of the unreactedtrimethoxyalkoxide-substituted vinylsilane in the pellets is about 1000to 1500 ppm.

The silane graft-modified polyolefins obtained by this method are shownas the “Silane-modified polyethylene” in Table 11 and Table 12.

[Method for Producing Modified PE with Functional Groups Other thanSilane-Modified PE, and its Copolymer]

Modified PE with functional groups other than silane-modified PE, andits copolymer, were produced by the following method.

All of the starting materials used were adjusted in molecular weight toan MI in the range of 0.5 to 10. Hydroxyl group-containing modified PEwas produced by saponification and neutralization of an EVA copolymer.For an amine-modified or oxazoline-modified resin, a tungsten-basedcatalyst is allowed to react with the terminal vinyl groups of PEpolymerized using a chromium catalyst, in the presence of hydrogenperoxide, for conversion of the vinyl groups to epoxy groups. Knownfunctional group-converting organic reactions were then used to convertthe respective reactive sites to the target functional groups, obtainingdifferent modified PE molecules. For amine-modified PE, for example,modified PE with epoxy groups is melt kneaded in an extruder at 200° C.while loading a primary or secondary amine in a liquid, and reaction iscarried out. The unreacted amine is then removed through a pressurereducing valve and the obtained amine-modified resin is extruded into astrand and cut into pellets.

The modified PE obtained by this method is indicated as “modified PE orcopolymer (B)” in Table 11 and Table 12.

Example III-1

To 79.2 weight % of polyethylene homopolymer (A) with a weight-averagemolecular weight of 500,000 there was added 19.8 weight % ofsilane-grafted polyethylene (PE (B)) with an MFR of 0.4 g/min, obtainedusing a polyolefin with a viscosity-average molecular weight of 20,000as starting material and modification reaction withtrimethoxyalkoxide-substituted vinylsilane (the respective contents ofresin compositions (A) and (B) thus being 0.8 and 0.2), and 1 weight %ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]as an antioxidant, and these were dry blended using a tumbler blender toobtain a mixture. The obtained mixture was supplied to a twin-screwextruder through a feeder, under a nitrogen atmosphere. Also, liquidparaffin (kinematic viscosity at 37.78° C.: 7.59×10⁻⁵ m²/s) was injectedinto the extruder cylinder by a plunger pump.

The mixture was melt kneaded with liquid paraffin in an extruder, andadjusted with a feeder and pump so that the quantity ratio of liquidparaffin in the extruded polyolefin composition was 70 weight % (i.e. apolymer concentration of 30 weight %). The melt kneading conditions werea preset temperature of 220° C., a screw rotational speed of 240 rpm anda discharge throughput of 18 kg/h.

The melt kneaded mixture was then extrusion cast through a T-die onto acooling roll controlled to a surface temperature of 25° C., to obtain agel sheet (molded sheet) with a raw membrane thickness of 1400 μm.

The molded sheet was then simultaneously fed into a biaxial tenterstretching machine for biaxial stretching, to obtain a stretched sheet.The stretching conditions were an MD factor of 7.0, a TD factor of 6.0(i.e. a factor of 7×6), and a biaxial stretching temperature of 125° C.

The stretched gel sheet was subsequently fed into a methyl ethyl ketonetank and thoroughly immersed in the methyl ethyl ketone for extractionremoval of the liquid paraffin, after which the methyl ethyl ketone wasdried off to obtain a porous body.

The porous body to be subjected to heat setting (HS) was fed to a TDtenter and HS was carried out at a heat setting temperature of 125° C.and a stretch ratio of 1.8, after which relaxation was carried out to afactor of 0.5 in the TD direction (i.e. the HS relaxation factor was0.5), to obtain a microporous membrane.

The obtained microporous membrane was then cut at the edges and wound upas a mother roll with a width of 1,100 mm and a length of 5,000 m.

During the evaluation, the microporous membrane wound out from themother roll was slit as necessary for use as the evaluation separator A.

Separators and batteries for evaluation were evaluated by the evaluationmethods described above, and the evaluation results are shown in Table11.

Examples III-2 to III-18

The microporous membranes and batteries listed in Table 11 and Table 12were obtained by the same procedure as Example III-1, except forchanging the types and quantity ratio of resins A and B and thecrosslinking method and conditions, as shown in Table 11 and Table 12.The obtained microporous membranes and batteries were subjected to eachevaluation by the evaluation methods described above, and the evaluationresults are shown in Table 11 and Table 12. For loading of theelectrolyte solutions in Examples III-8 to III-10 and III-15 to III-18,solutions of the additives listed in Table 11 and Table 12 dissolved insuitable amounts of electrolyte solutions were used.

Comparative Examples III-1 and III-2

The microporous membranes listed in Table 12 were obtained by the sameprocedure as Example III-1, except for changing the types and quantityratio of resins A and B and the crosslinking method and conditions asshown in Table 12. The obtained microporous membranes were used forelectron beam crosslinking by irradiation at a prescribed radiationdose. The obtained electron beam-crosslinked microporous membranes andbatteries were evaluated by each of the evaluation methods describedabove, and the evaluation results are shown in Table 12.

FIG. 8 shows a strain-crystal fragmentation rate graph for ComparativeExample III-2 and Example III-1, where the change in X-ray crystalstructure during a tensile rupture fracture test was observed. In FIG. 8, the microporous membrane of Comparative Example III-2 is representedas the dotted line “EB crosslinked”, and the microporous membrane ofExample III-1 is represented as the solid line “before chemicalcrosslinking” and the dashed line “after chemical crosslinking”.

TABLE 11A Example III 1 2 3 4 5 Microporous Resin PE (A) 80 80 80 80 80membrane composition Modified PE or Silane-modified 20 — — — — (weight%) copolymer (B)* polyethylene —COOH modified — 20 10 — — PE-oxazoline-modified — — — — 10 PE -oxazoline, —OH — — — 20 — modified PE—OH modified PE — — 10 — 10 —OH, —NH— — — — — — modified PE —OH, amine-— — — — — modified PE Crosslinking Method — — — — — TimingApparatus/conditions Basic Membrane thickness μm 11 11 11 11 11separator Porosity (i) % 40 39 39 40 40 properties Air permeabilitysec/100 cm³ 160 170 170 153 153 Resin aggregates /1000 m² 2 3 3 7 7 inseparator Storage modulus R_(E′x) Factor 2.2 2.5 2.5 2.3 2.3 changefactor, ver. 2 R_(E′mix) Factor 8.4 7.5 7.5 7.5 7.6 Loss modulus R_(E″x)Factor 2 2.4 2.4 0.21 2.1 change factor, ver. 2 R_(E″mix) Factor 6.1 6.56.5 6.5 6.9 F/MD Fuse ° C. 141 142 142 143 143 property (i) temperatureMeltdown ° C. >200 >200 >200 >200 >200 temperature Battery CrosslinkingMethod** I II II II II Reaction/bonding Dehydrating EsterificationEsterification Amide Amide condensation bond, bond, ether ether bondbond Timing Contact Contact Contact Contact Contact with with with withwith electrolyte electrolyte electrolyte electrolyte electrolytesolution solution solution solution solution until until until untiluntil initial initial initial initial initial charge- charge- charge-charge- charge- discharge discharge discharge discharge dischargeFunctional A silanol group —OH —OH oxazoline oxazoline group ofmicroporous B — —COOH —COOH —OH —OH membrane Reactive species — — — — —Catalyst type HF — — — — Molten metal species — — — — — Additive**** — —— — — Battery cycle stability 1 % 98 97 97 93 93 Battery Internal °C./sec 5 6 6 5 5 destruction maximum safety 1 heat release rate Voltagesec >300 >300 >300 >300 >300 reduction (3 V reduction time) Example III6 7 8 9 10 Microporous Resin PE (A) 80 80 80 80 80 membrane compositionModified PE or Silane-modified — — — — — (weight %) copolymer (B)*polyethylene —COOH modified — — — — — PE -oxazoline-modified — — — — —PE -oxazoline, —OH — — — — — modified PE —OH modified PE 20 — — 20 20—OH, —NH— — 20 — — — modified PE —OH, amine- — — 20 — — modified PECrosslinking Method — — — — — Timing Apparatus/conditions Basic Membranethickness μm 11 11 11 11 11 separator Porosity (i) % 42 41 38 39 40properties Air permeability sec/100 cm³ 166 158 162 172 150 Resinaggregates /1000 m² 5 3 2 5 2 in separator Storage modulus R_(E′x)Factor 2.6 3.4 3.5 2.2 2.2 change factor, ver. 2 R_(E′mix) Factor 8.16.4 6.4 7.5 7.5 Loss modulus R_(E″x) Factor 2.3 3.1 3.3 2.1 2.1 changefactor, ver. 2 R_(E″mix) Factor 7.8 5.6 5.6 6.4 6.4 F/MD Fuse ° C. 143144 143 142 141 property (i) temperature Meltdown °C. >200 >200 >200 >200 >200 temperature Battery Crosslinking Method**III III IV IV IV Reaction/bonding Chain Chain Nucleophilic NucleophilicEpoxy condensation condensed substitution addition ring —O—CO—O—tertiary opening amine Timing Contact Contact Contact Contact Contactwith with with with with electrolyte electrolyte electrolyte electrolyteelectrolyte solution solution solution solution solution until untiluntil initial initial initial charge- charge- charge- dischargedischarge discharge Functional A —OH —NH— —NH₂ —OH —OH group ofmicroporous B — — — — — membrane Reactive species EC*** EC*** — — —Catalyst type — — — — — Molten metal species — — — — — Additive**** — —BS (PEG)₅ diisocyanate diepoxy compound Battery cycle % 92 91 92 96 95stability 1 Battery Internal ° C./sec 7 5 6 8 4 destruction maximumsafety 1 heat release rate Voltage sec >300 >300 >300 >300 >300reduction (3 V reduction time)

TABLE 11B Example III 1 2 3 4 5 Separator Maximum elastic Storage MPa280 350 550 9300 2350 solid modulus at −50 modulus (E′) viscoelasticityto 250° C. Loss MPa 75 52 200 23 230 ver. 3 data modulus (E″) Minimumelastic Storage MPa 2.21 3.5 3.1 2.5 5.2 modulus, −50 modulus (E′) to250° C. Loss MPa 0.88 1.3 1.2 1.2 1.7 modulus (E″) Maximum elasticStorage MPa 7.2 8.1 11 15 9.2 modulus, membrane modulus (E′) softeningtransition Loss MPa 2.15 6.2 7.6 3.5 3.1 temperature to 250° C. modulus(E″) Minimum elastic Storage MPa 2.21 3.5 3.1 2.5 5.2 modulus, membranemodulus (E′) softening transition Loss MPa 0.88 1.3 1.2 1.2 1.7temperature to 250° C. modulus (E″) Membrane softening Mean storage MPa4.7 5.8 7.1 8.8 7.2 transition temperature modulus (E′ave) -membranerupture Mean loss MPa 1.5 3.8 4.4 2.4 2.4 temperature modulus (E″ave)Membrane softening transition temperature ° C. 147 149 148 146 146Membrane rupture temperature ° C. No No No No No membrane membranemembrane membrane membrane rupture rupture rupture rupture rupture at250 at 250 at 250 at 250 at 250 Example III 6 7 8 9 10 Separator Maximumelastic Storage MPa 3330 4860 81 250 2570 solid modulus at −50 modulus(E′) viscoelasticity to 250° C. Loss MPa 71 1080 14 24 108 ver. 3 datamodulus (E″) Minimum elastic Storage MPa 4.6 2.24 3.5 7.5 7.2 modulus,−50 modulus (E′) to 250° C. Loss MPa 1.1 1.4 1.1 5.5 6.6 modulus (E″)Maximum elastic Storage MPa 11.1 17.3 8.5 9 13.1 modulus, membranemodulus (E′) softening transition Loss MPa 1.27 2.1 5.1 8.1 9.1temperature to 250° C. modulus (E″) Minimum elastic Storage MPa 4.6 2.243.5 7.5 7.2 modulus, membrane modulus (E′) softening transition Loss MPa1.1 1.4 1.1 5.5 6.6 temperature to 250° C. modulus (E″) Membranesoftening Mean storage MPa 7.9 9.8 6.0 8.3 10.2 transition temperaturemodulus (E′ave) -membrane rupture Mean loss MPa 1.2 1.8 3.1 6.8 7.9temperature modulus (E″ave) Membrane softening transition temperature °C. 145 148 149 147 145 Membrane rupture temperature ° C. No No No No Nomembrane membrane membrane membrane membrane rupture rupture rupturerupture rupture at 250 at 250 at 250 at 250 at 250

TABLE 12A Example III 11 12 13 14 15 Microporous Resin PE (A) 80 80 9910 99 membrane composition Modified PE or Silane-modified — — 1 90 —(weight %) copolymer (B)* polyethylene —COOH modified 20 20 — — — PE-oxazoline-modified — — — — — PE -oxazoline, —OH — — — — — modified PE—OH modified PE — — — — 1 —OH, —NH— — — — — — modified PE —OH, amine- —— — — — modified PE Crosslinking Method — — — — — TimingApparatus/conditions Basic Membrane thickness μm 11 11 11 11 11separator Porosity (i) % 40 41 42 41 37 properties Air permeabilitysec/100 cm³ 136 153 155 161 148 Resin aggregates /1000 m² 3 4 1 30 2 inseparator Storage modulus R_(E′x) Factor 2.3 2.4 1.6 18 2 change factor,ver. 2 R_(E′mix) Factor 7.8 7.5 2.5 19 2.3 Loss modulus R_(E″x) Factor2.1 2.2 1.6 17.3 1.9 change factor, ver. 2 R_(E″mix) Factor 6.3 6.2 2.318.6 2.1 F/MD Fuse ° C. 142 141 145 148 146 property (i) temperatureMeltdown ° C. >200 >200 >200 >200 >200 temperature Battery CrosslinkingMethod** V V I I IV Reaction/bonding Coordination CoordinationDehydrating Dehydrating Nucleophilic bonding bonding condensationcondensation addition Timing Contact Contact Contact Contact Contactwith with with with with electrolyte electrolyte electrolyte electrolyteelectrolyte solution solution solution solution solution until untiluntil until until initial initial initial initial initial charge-charge- charge- charge- charge- discharge discharge discharge dischargedischarge Functional A —OH —OH silanol silanol —OH group of group groupmicroporous B —COOH —COOH — — — membrane Reactive species — — — — —Catalyst type HF, H₂O HF, H₂O HF HF — Molten metal species Ni²⁺ Li⁺ — —— Additive**** — — — — Diisocyanate Battery cycle stability 1 % 97 91 8283 76 Battery Internal ° C./sec 5 7 23 7 26 destruction maximum safety 1heat release rate Voltage sec >300 >300 280 >300 285 reduction (3 Vreduction time) Comparative Example III Example III 16 17 18 1 2Microporous Resin PE (A) 10 99.7 0.03 100 100 membrane compositionModified PE or Silane-modified — — — — — (weight %) copolymer (B)*polyethylene —COOH modified — — — — — PE -oxazoline-modified — — — — —PE -oxazoline, —OH — — — — — modified PE —OH modified PE 90 0.3 99.7 — ——OH, —NH— — — — — — modified PE —OH, amine- — — 1 — — modified PECrosslinking Method — — — Electron Electron beam beam irradiationirradiation Timing After After membrane membrane formation - formation -before before battery battery assembly assembly Apparatus/conditions EBEB apparatus/ apparatus/ 20 kGy 120 kGy Basic Membrane thickness μm 1111 11 11 11 separator Porosity (i) % 39 29 37 37 37 properties Airpermeability sec/100 cm³ 153 156 181 162 163 Resin aggregates /1000 m²38 11 203 2 2 in separator Storage modulus R_(E′x) Factor 18.3 1.2 211.1 21.5 change factor, ver. 2 R_(E′mix) Factor 19.5 1.4 22 1.1 22 Lossmodulus R_(E″x) Factor 17.8 1.1 21 1.1 21 change factor, ver. 2R_(E″mix) Factor 18.6 1.2 22.5 1.1 21.5 F/MD Fuse ° C. 148 146 148 155182 property (i) temperature Meltdown ° C. >200 >200 >200 158 >200temperature Battery Crosslinking Method** IV IV IV — — Reaction/bondingNucleophilic Nucleophilic Nucleophilic addition addition addition TimingContact Contact Contact with with with electrolyte electrolyteelectrolyte solution solution solution until until until initial initialinitial charge- charge- charge- discharge discharge discharge FunctionalA —OH —OH —OH group of microporous B — — — membrane Reactive species — —— Catalyst type — — — Molten metal species — — — Additive****Diisocyanate Diisocyanate Diisocyanate Battery cycle stability 1 % 79 6361 54 43 Battery Internal ° C./sec 6 35 38 127 57 destruction maximumsafety 1 heat release rate Voltage sec >300 253 222 3 12 reduction (3 Vreduction time)

TABLE 12B Example III 11 12 13 14 15 16 Separator Maximum elasticStorage MPa 9,680 225 120 230 112 8650 solid modulus at −50 modulus (E′)viscoelasticity to 250° C. Loss MPa 7,880 15 23 35 15 7250 ver. 3modulus (E″) data Minimum elastic Storage MPa 1.2 9.1 3.2 9.6 9.2 9.8modulus, −50 modulus (E′) to 250° C. Loss MPa 0.2 0.5 1.3 7.3 4.1 8.5modulus (E″) Maximum elastic Storage MPa 5.3 14.4 6.6 13.5 16.2 13.7modulus, membrane modulus (E′) softening transition Loss MPa 3.2 1.3 3.19.4 5.6 10.3 temperature to 250° C. modulus (E″) Minimum elastic StorageMPa 1.2 9.1 3.2 9.6 9.2 9.8 modulus, membrane modulus (E′) softeningtransition Loss MPa 0.2 0.5 1.3 7.3 4.1 8.5 temperature to 250° C.modulus (E″) Membrane softening Mean storage MPa 3.3 11.8 4.9 11.6 12.711.8 transition temperature modulus (E′ave) membrane rupture Mean lossMPa 1.7 0.9 2.2 8.4 4.9 9.4 temperature modulus (E″ave) Membranesoftening transition temperature ° C. 146 148 147 147 144 141 Membranerupture temperature ° C. No No 179 No 178 No membrane membrane membranemembrane rupture rupture rupture rupture at 250 at 250 at 250 at 250Comparative Example III Example III 17 18 1 2 Separator Maximum elasticStorage MPa 9,700 124 87 13,820 solid modulus at −50 modulus (E′)viscoelasticity to 250° C. Loss MPa 7,890 13 8.3 10,135 ver. 3 modulus(E″) data Minimum elastic Storage MPa 9.6 9.7 0.8 20.8 modulus, −50modulus (E′) to 250° C. Loss MPa 0.2 6.9 0.09 17.3 modulus (E″) Maximumelastic Storage MPa 15.2 16.6 0.1 23 modulus, membrane modulus (E′)softening transition Loss MPa 1.4 14.1 0.03 20.8 temperature to 250° C.modulus (E″) Minimum elastic Storage MPa 9.6 9.7 0.8 20.8 modulus,membrane modulus (E′) softening transition Loss MPa 0.2 6.9 0.09 17.3temperature to 250° C. modulus (E″) Membrane softening Mean storage MPa12.4 13.2 0.5 21.9 transition temperature modulus (E′ave) membranerupture Mean loss MPa 0.8 10.5 0.1 19.1 temperature modulus (E″ave)Membrane softening transition temperature ° C. 145 149 151 156 Membranerupture temperature ° C. 185 No 161 No membrane membrane rupture ruptureat 250 at 250Explanation of Abbreviations in Table 11 and Table 12

*The “silane-modified polyethylene” is a silane-modified polyethylenewith a density of 0.95 g/cm³ and a melt mass-flow rate (MFR) of 0.4g/min at 190° C., obtained by modification reaction with atrimethoxyalkoxide-substituted vinylsilane, using a polyolefin with aviscosity-average molecular weight of 20,000 as the starting material.

The abbreviations “—COOH modified PE”, “-oxazoline modified PE”,“-oxazoline, —OH modified PE”, “—OH modified PE”, “—OH, —NH— modifiedPE” and “—OH, amine modified PE” are the modified PE molecules obtainedby the [Method for producing modified PE with functional groups otherthan silane-modified PE, and its copolymer], above.

**(I) Condensation reaction between multiple identical functional groups

(II) Reaction between multiple different functional groups

(III) Chain condensation reaction between functional groups andelectrolyte solution

(IV) Reaction between functional groups and additives

(V) Reaction in which multiple identical functional groups arecrosslinked by coordination bonding with eluting metal ions

***EC: Ethylene carbonate

****BS(PEG)₅: Both terminal succinimides, EO unit repeats: 5

Diisocyanate: Compound with both terminal isocyanates linked with hexaneunits linked via urethane bonding

Diepoxy compound: Compound with both terminal epoxide groups and butaneunits linked

<Experiment Group IV>

Example IV-1

<Fabrication of Layer A>

(Production of Silane Graft-Modified Polyolefin)

Using polyethylene with a viscosity-average molecular weight of 120,000as the polyethylene, the polyethylene starting material was melt kneadedwith an extruder while adding an organic peroxide (di-t-butyl peroxide)and generating radicals in the polymer chain of the α-olefin, and thenit was filled with trimethoxyalkoxide-substituted vinylsilane andaddition reaction was carried out to introduce alkoxysilyl groups intothe α-olefin polymer, forming a silane-graft structure. A suitableamount of an antioxidant(pentaerythritoltetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate])was simultaneously added to the reaction system to adjust the radicalconcentration in the system, thus inhibiting chain-style chain reaction(gelation) in the α-olefin. The obtained silane-grafted polyolefinmolten resin was cooled in water and pelletized, after which it washeat-dried at 80° C. for 2 days and the water and unreactedtrimethoxyalkoxide-substituted vinylsilane were removed. The residualconcentration of the unreacted trimethoxyalkoxide-substitutedvinylsilane in the pellets was about 1500 ppm or lower.

Modification reaction of the trimethoxyalkoxide-substituted vinylsilanein this manner yielded a silane-modified polyethylene with an MFR (190°C.) of 0.4 g/min.

(Fabrication of Layer A)

After combining 35 weight % of the previously obtained silane-modifiedpolyethylene obtained with 65 weight % of polyethylene homopolymer witha weight-average molecular weight of 800,000 to obtain a resin blend, 1weight % ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]was added to the blend as an antioxidant, and a tumbler blender was usedfor dry blending to obtain a mixture. The obtained mixture was suppliedto a twin-screw extruder through a feeder, under a nitrogen atmosphere.Also, liquid paraffin (kinematic viscosity at 37.78° C.: 7.59×10⁻⁵ m²/s)was injected into the extruder cylinder by a plunger pump.

The mixture was melt kneaded with liquid paraffin in an extruder, andadjusted with a feeder and pump so that the quantity ratio of liquidparaffin in the extruded polyolefin composition was 70 weight % (i.e. apolymer concentration of 30 weight %). The melt kneading conditions werea preset temperature of 220° C., a screw rotational speed of 240 rpm anda discharge throughput of 18 kg/h. The melt kneaded mixture was thenextrusion cast through a T-die onto a cooling roll controlled to asurface temperature of 25° C., to obtain a gel sheet (molded sheet) witha raw membrane thickness of 1400 μm.

The molded sheet was then simultaneously fed into a biaxial tenterstretching machine for biaxial stretching, to obtain a stretched sheet.The stretching conditions were an MD factor of 7.0, a TD factor of 6.3(i.e. a factor of 7×6.3), and a biaxial stretching temperature of 122°C.

The stretched gel sheet was then fed into a dichloromethane tank andthoroughly immersed in the dichloromethane for extraction removal of theliquid paraffin, after which the dichloromethane was dried off to obtaina porous body.

The porous body to be subjected to heat setting (HS) was fed to a TDtenter and HS was carried out at a heat setting temperature of 133° C.and a stretch ratio of 1.8, after which relaxation was carried out to afactor of 1.7 in the TD direction to obtain a microporous membrane.

The obtained microporous membrane was then cut at the edges and wound upas a mother roll with a width of 1,100 mm and a length of 5,000 m.

During the evaluation, the microporous membrane wound out from themother roll was slit as necessary for use as the evaluation layer A.

The membrane thickness, air permeability and porosity were measured forthe obtained evaluation layer A, and they are shown in Table 13.

<Fabrication of Layer B>

A dispersion was prepared by evenly dispersing 95 parts by weight ofaluminum hydroxide oxide (mean particle size: 1.4 μm) as inorganicparticles and 0.4 part by (solid) weight of an aqueous ammoniumpolycarboxylate solution (SN dispersant 5468 by San Nopco, Ltd., 40%solid concentration) as an ionic dispersing agent, in 100 parts byweight of water. The obtained dispersion was shredded with a bead mill(cell volume: 200 cc, zirconia bead diameter: 0.1 mm, filling volume:80%), and the particle size distribution of the inorganic particles wasadjusted to D50=1.0 μm, to prepare an inorganic particle-containingslurry.

The microporous membrane was then continuously wound out from themicroporous membrane mother roll and one side of the microporousmembrane was coated with the inorganic particle-containing slurry usinga gravure reverse coater, after which it was dried with a dryer at 60°C. to remove the water and wound up to obtain a separator mother roll.

During the evaluation, the separator was wound out from the mother rolland slit as necessary for use as the evaluation separator.

Examples IV-2 to IV-5 and Comparative Examples IV-1 to IV-21

With the physical properties listed in Table 13 as the target, one ormore from among the weight-average molecular weight of the polyethylenehomopolymer, the set stretching conditions, the heat setting conditionsand the relaxation conditions were changed. The composition of layer Bwas also changed as shown in Table 13.

Separators were fabricated by the same method as Example IV-1 except forthese changes, and the obtained separators were used for the evaluationdescribed above. The evaluation results are shown in Table 13.

TABLE 13 Example IV 1 2 3 4 Separator Layer A Polyethylene weight % 6540 95 65 Silane-modified weight % 35 60 5 35 polyolefin Thickness (TA)um 11.0 3.0 11.0 16.0 Porosity (iii) % 40 38 42 43 Air permeabilitysec/cm³ 155 105 107 200 Puncture strength gf/20 um 490 380 505 168 LayerB Inorganic Weight wt % 95.00 94.00 35.00 98.00 ratio particles Type —AlO(OH) AlO(OH) AlO(OH) AlO(OH) Thickness (TB) um 3.5 12 3.5 0.5 Ratio(TA/TB) — 3.14 0.25 3.14 32.00 Total thickness (TA + TB) um 14.5 15 14.516.50 TMA test Membrane rupture ° C. 210.00 211.00 207.00 168.00temperature F/MD test (ii) Shutdown temperature ° C. 143 140 143 138Meltdown temperature ° C. 219 220 220 182 150° C. Before formation of %56 63 55 65 heat crosslinked structure shrinkage After formation of % 71.2 48 58 factor crosslinked structure Change factor Factor 0.13 0.020.87 0.892 Battery Battery cycle stability 2 (300 cycles) % 98 81 93 67Passing rate in  200 cycles % 97 83 97 50 safety test 2 1000 cycles % 9060 83 5 Comparative Example IV Example IV 5 1 2 Separator Layer APolyethylene weight % 65 95 40 Silane-modified weight % 35 5 60polyolefin Thickness (TA) um 3.0 16.0 3.0 Porosity (iii) % 52 42 40 Airpermeability sec/cm³ 215 205 189 Puncture strength gf/20 um 168.5 168168.5 Layer B Inorganic Weight wt % 88.00 98.00 99.00 ratio particlesType — AlO(OH) AlO(OH) AlO(OH) Thickness (TB) um 20 0.5 20 Ratio (TA/TB)— 0.15 32.00 0.15 Total thickness (TA + TB) um 23.00 16.50 23.00 TMAtest Membrane rupture ° C. 158.00 168.00 158.00 temperature F/MD test(ii) Shutdown temperature ° C. 139 136 158 Meltdown temperature ° C. 217182 217 150° C. Before formation of % 38 65 43 heat crosslinkedstructure shrinkage After formation of % 32 60 0.5 factor crosslinkedstructure Change factor Factor 0.842 0.923 0.012 Battery Battery cyclestability 2 (300 cycles) % 66 67 66 Passing rate in  200 cycles % 51 4950 safety test 2 1000 cycles % 7 7 9<Experiment Group V>[Silane Graft-Modified Polyolefin Production Method]

The polyolefin starting material to be used as the silane graft-modifiedpolyolefin may be one with a viscosity-average molecular weight (Mv) of100,000 to 1,000,000, a weight-average molecular weight (Mw) of 30,000to 920,000, and a number-average molecular weight of 10,000 to 150,000,and it may be propylene or a butene-copolymerized α-olefin. After meltkneading the polyethylene starting material with an extruder whileadding an organic peroxide (di-t-butyl peroxide) and generating radicalsin the polymer chain of the α-olefin, it is filled withtrimethoxyalkoxide-substituted vinylsilane and addition reaction iscarried out to introduce alkoxysilyl groups into the α-olefin polymer,forming a silane-graft structure. A suitable amount of an antioxidant(pentaerythritoltetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate])is simultaneously added to adjust the radical concentration in thesystem, thus inhibiting chain-style chain reaction (gelation) in theα-olefin. The obtained silane-grafted polyolefin molten resin is cooledin water and pelletized, after which it is heat-dried at 80° C. for 2days and the water and unreacted trimethoxyalkoxide-substitutedvinylsilane are removed. The residual concentration of the unreactedtrimethoxyalkoxide-substituted vinylsilane in the pellets is about 10 to1500 ppm.

The silane graft-modified polyolefins obtained by this method are usedas the “Silane-modified polyethylene (B)” in Tables 14 to 16. The silanegraft-modified polyolefins used here have a density of 0.94 g/cm³ and anMFR of 0.65 g/min.

Example V-1

(Formation of Microporous Membrane)

To 79.2 wt % of polyethylene homopolymer with a weight-average molecularweight of 500,000 (A) there was added 19.8 wt % of silane-graftedpolyethylene (silane-modified polyethylene (B)) with an MFR (190° C.) of0.4 g/min, obtained using a polyolefin with a viscosity-averagemolecular weight of 20,000 as starting material and modificationreaction with trimethoxyalkoxide-substituted vinylsilane (the respectivecontents of resin compositions (A) and (B) thus being 80% and 20%), and1 wt % ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]as an antioxidant, and these were dry blended using a tumbler blender toobtain a mixture. The obtained mixture was supplied to a twin-screwextruder through a feeder, under a nitrogen atmosphere. Also, liquidparaffin (kinematic viscosity at 37.78° C.: 7.59×10⁻⁵ m²/s) was injectedinto the extruder cylinder by a plunger pump.

The mixture was melt kneaded with liquid paraffin in an extruder, andadjusted with a feeder and pump so that the quantity ratio of liquidparaffin in the extruded polyolefin composition was 70 wt % (i.e. apolymer concentration of 30 wt %). The melt kneading conditions were apreset temperature of 220° C., a screw rotational speed of 240 rpm and adischarge throughput of 18 kg/h.

The melt kneaded mixture was then extrusion cast through a T-die onto acooling roll controlled to a surface temperature of 25° C., to obtain agel sheet (molded sheet) with a raw membrane thickness of 1400 μm.

The molded sheet was then simultaneously fed into a biaxial tenterstretching machine for biaxial stretching, to obtain a stretched sheet.The stretching conditions were an MD factor of 7.0, a TD factor of 6.0(i.e. a factor of 7×6), and a biaxial stretching temperature of 125° C.

The stretched gel sheet was subsequently fed into a methyl ethyl ketonetank and thoroughly immersed in the methyl ethyl ketone for extractionremoval of the liquid paraffin, after which the methyl ethyl ketone wasdried off to obtain a porous body.

The porous body to be subjected to heat setting (HS) was fed to a TDtenter and HS was carried out at a heat setting temperature of 125° C.and a stretch ratio of 1.8, after which relaxation was carried out to afactor of 0.5 in the TD direction (i.e. the HS relaxation factor was0.5), to obtain a microporous membrane.

The obtained microporous membrane was then cut at the edges and wound upas a microporous membrane mother roll with a width of 1,100 mm and alength of 5,000 m.

(Method for Producing Acrylic Latex)

The acrylic latex to be used as the resin binder is produced by thefollowing method.

Into a reactor equipped with a stirrer, reflux condenser, drip tank andthermometer there were loaded 70.4 parts by weight of ion-exchangedwater, 0.5 part by weight of “AQUALON KH1025” (registered trademark ofDai-ichi Kogyo Seiyaku Co., Ltd., 25% aqueous solution) as anemulsifier, and 0.5 part by weight of “ADEKA REASOAP SR1025” (registeredtrademark of Adeka Corp., 25% aqueous solution). The internaltemperature of the reactor was then raised to 80° C., and 7.5 parts byweight of a 2% aqueous solution of ammonium persulfate was added whilekeeping the temperature at 80° C., to obtain an initial mixture. Fiveminutes after addition of the ammonium persulfate aqueous solution wascompleted, the emulsified liquid was added dropwise from the drip tankinto the reactor over a period of 150 minutes.

The emulsified liquid was prepared by forming a mixture of 70 parts byweight of butyl acrylate; 29 parts by weight of methyl methacrylate; 1part by weight of methacrylic acid; 3 parts by weight of “AQUALONKH1025” (registered trademark of Dai-ichi Kogyo Seiyaku Co., Ltd., 25%aqueous solution) and 3 parts by weight “ADEKA REASOAP SR1025”(registered trademark of Adeka Corp., 25% aqueous solution) asemulsifiers; 7.5 parts by weight of a 2% aqueous solution of ammoniumpersulfate; and 52 parts by weight of ion-exchanged water, and mixing itwith a homomixer for 5 minutes.

Upon completion of the dropwise addition of the emulsified liquid, theinternal temperature of the reactor was kept at 80° C. for a period of90 minutes, after which it was cooled to room temperature. The obtainedemulsion was adjusted to a pH of 8.0 with a 25% aqueous ammoniumhydroxide solution, and then a small amount of water was added to obtainan acrylic latex with a solid content of 40%. The obtained acrylic latexhad a number-mean particle size of 145 nm and a glass transitiontemperature of −23° C.

(Formation of Inorganic Porous Layer)

A dispersion was prepared by evenly dispersing 95 parts by weight ofaluminum hydroxide oxide (mean particle size: 1.4 μm) as inorganicparticles and 0.4 part by (solid) weight of an aqueous ammoniumpolycarboxylate solution (SN dispersant 5468 by San Nopco, Ltd., 40%solid concentration) as an ionic dispersing agent, in 100 parts byweight of water. The obtained dispersion was shredded with a bead mill(cell volume: 200 cc, zirconia bead diameter: 0.1 mm, filling volume:80%), and the particle size distribution of the inorganic particles wasadjusted to D50=1.0 μm. To the particle size distribution-adjusteddispersion there was added 4.6 parts by (solid) weight of an acryliclatex (solid concentration: 40%, mean particle size: 145 nm, glasstransition temperature: −23° C., constituent monomers: butyl acrylate,methyl methacrylate, methacrylic acid) as a resin binder to prepare aninorganic particle-containing slurry.

The microporous membrane was then continuously wound out from themicroporous membrane mother roll and one side of the microporousmembrane was coated with the inorganic particle-containing slurry usinga gravure reverse coater, after which it was dried with a dryer at 60°C. to remove the water and wound up to obtain a separator mother roll.

During the evaluation, the separator was wound out from the mother rolland slit as necessary for use as the evaluation separator.

Examples V-2 to V-12 and Comparative Example V-2

The microporous membranes listed in Tables 14 to 16 were obtained by thesame procedure as Example V-1, except for changing the quantity ratio ofcomponents A and B, the presence or absence of the inorganic layer andthe crosslinking method and conditions, as shown in Tables 14 to 16.

Comparative Example V-1

To 79.2 wt % of polyethylene homopolymer with a weight-average molecularweight of 500,000 (A) there was added 19.8 wt % of silane-graftedpolyethylene (silane-modified polyethylene (B)) with an MFR (190° C.) of0.4 g/min, obtained using a polyolefin with a viscosity-averagemolecular weight of 20,000 as starting material and modificationreaction with trimethoxyalkoxide-substituted vinylsilane (the respectivecontents of resin compositions (A) and (B) thus being 80% and 20%), and1 wt % ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]as an antioxidant, and these were dry blended using a tumbler blender toobtain a mixture. The obtained mixture was supplied to a twin-screwextruder through a feeder, under a nitrogen atmosphere. Also, liquidparaffin (kinematic viscosity at 37.78° C.: 7.59×10⁻⁵ m²/s) was injectedinto the extruder cylinder by a plunger pump.

The mixture was melt kneaded with liquid paraffin in an extruder, andadjusted with a feeder and pump so that the quantity ratio of liquidparaffin in the extruded polyolefin composition was 70 wt % (i.e. apolymer concentration of 30 wt %). The melt kneading conditions were apreset temperature of 220° C., a screw rotational speed of 240 rpm and adischarge throughput of 18 kg/h.

The melt kneaded mixture was then extrusion cast through a T-die onto acooling roll controlled to a surface temperature of 25° C., to obtain agel sheet (molded sheet) with a raw membrane thickness of 1400 μm.

The molded sheet was then simultaneously fed into a biaxial tenterstretching machine for biaxial stretching, to obtain a stretched sheet.The stretching conditions were an MD factor of 7.0, a TD factor of 6.0(i.e. a factor of 7×6), and a biaxial stretching temperature of 125° C.

The stretched gel sheet was subsequently fed into a methyl ethyl ketonetank and thoroughly immersed in the methyl ethyl ketone for extractionremoval of the liquid paraffin, after which the methyl ethyl ketone wasdried off to obtain a porous body.

The porous body to be subjected to heat setting (HS) was fed to a TDtenter and HS was carried out at a heat setting temperature of 125° C.and a stretch ratio of 1.8, after which relaxation was carried out to afactor of 0.5 in the TD direction (i.e. the HS relaxation factor was0.5).

In order to use the heat-treated porous body as the separator inComparative Example V-1, the obtained porous body was cut at the edgesand wound up as a mother roll with a width of 1,100 mm and a length of5,000 m.

During the evaluation for Comparative Example V-1, the microporousmembrane wound out from the mother roll was slit as necessary for use asthe evaluation separator.

[Evaluation Results]

The microporous membranes and batteries obtained in Examples V-1 to V-12and Comparative Examples V-1 and V-2 were evaluated by each of theevaluation methods described above, and the evaluation results are shownin Tables 14 to 16.

TABLE 14 Exam- Exam- Exam- ple V-1 ple V-2 ple V-3 Separator ResinPolyethylene (A) wt % 80% 80% 80% composition Silane-modified wt % 20%20% 20% polyethylene (B) Kneading temperature ° C. 220 220 220 Inorganiclayer Inorganic Weight 95% 95% 95% composition particles ratio TypeAlO(OH) AlO(OH) AlO(OH) Resin binder Tg (° C.) −23 −23 −23 CrosslinkingMethod — — — method Crosslinking reaction timing Reagent Temperature °C. pH Basic properties Membrane thickness um 11 11 11 of resinousPorosity (ii) % 40 40 40 microporous Air permeability sec/100 cm³ 160160 160 membrane Inorganic layer thickness um 2 0.5 5 Shutdown/ruptureShutdown ° C. 143 143 143 resistance temperature (i) Membrane rupture °C. ≥200 ≥200 ≥200 temperature (i) Resin aggregates in microporousmembrane /1000 m² 2 2 2 Storage modulus R_(ΔE′) Factor 2.1 2.1 2.1change factor, ver. 1 R_(E′mix) Factor 8.5 8.5 8.5 Loss modulus R_(ΔE″)Factor 1.9 1.9 1.9 change factor, ver. 1 R_(E″mix) Factor 6.2 6.2 6.2Transition temperature ° C. 143 143 143 calculated by storage modulus,ver. 1 Battery Crosslinking method Contact Contact Contact with withwith electrolyte electrolyte electrolyte solution solution solutionuntil until until initial initial initial charge- charge- charge-discharge discharge discharge Battery cycle stability 1 % 98 96 95Battery Internal ° C./sec 5 7 5 destruction maximum safety 1 heatrelease rate Voltage sec >300 >300 >300 reduction (3 V reduction time)Exam- Exam- Exam- ple V-4 ple V-5 ple V-6 Separator Resin Polyethylene(A) wt % 80% 80% 80% composition Silane-modified wt % 20% 20% 20%polyethylene (B) Kneading temperature ° C. 220 220 220 Inorganic layerInorganic Weight  6% 98%  3% composition particles ratio Type AlO(OH)AlO(OH) AlO(OH) Resin binder Tg (° C.) −23 −23 −23 Crosslinking Method —— — method Crosslinking reaction timing Reagent Temperature ° C. pHBasic properties Membrane thickness um 11 11 11 of resinous Porosity(ii) % 40 40 40 microporous Air permeability sec/100 cm³ 160 160 160membrane Inorganic layer thickness um 4 4 4 Shutdown/rupture Shutdown °C. 143 143 143 resistance temperature (i) Membrane rupture ° C. ≥200≥200 ≥200 temperature (i) Resin aggregates in /1000 m² 2 2 2 microporousmembrane Storage modulus R_(ΔE′) Factor 2.1 2.1 2.1 change factor, ver.1 R_(E′mix) Factor 8.5 8.5 8.5 Loss modulus R_(ΔE″) Factor 1.9 1.9 1.9change factor, ver. 1 R_(E″mix) Factor 6.2 6.2 6.2 Transitiontemperature ° C. 143 143 143 calculated by storage modulus, ver. 1Battery Crosslinking method Contact Contact Contact with with withelectrolyte electrolyte electrolyte solution solution solution untiluntil until initial initial initial charge- charge- charge- dischargedischarge discharge Battery cycle stability 1 % 98 94 81 BatteryInternal ° C./sec 9 5 16 destruction maximum safety 1 heat release rateVoltage sec 293 >300 265 reduction (3 V reduction time)

TABLE 15 Example V-7 Example V-8 Example V-9 Separator ResinPolyethylene (A) wt % 80% 94% 62% composition Silane-modified wt % 20% 6% 38% polyethylene (B) Kneading temperature ° C. 220    220 220Inorganic layer Inorganic Weight 99.5%  95% 95% composition particlesratio Type AlO(OH) AlO(OH) AlO(OH) Resin binder Tg (° C.) −23    −23 −23Crosslinking method Method — — — Crosslinking reaction timing ReagentTemperature ° C. pH Basic properties Membrane thickness um 11   11 11 ofresinous microporous Porosity (ii) % 40   39 42 membrane Airpermeability sec/100 cm³ 160    167 173 Inorganic layer thickness um 4  4 4 Shutdown/rupture Shutdown ° C. 143    142 143 resistance temperature(i) Membrane rupture ° C. ≥200    ≥200 ≥200 temperature (i) Resinaggregates in microporous membrane /1000 m² 2   7 18 Storage modulusR_(ΔE′) Factor 2.1 1.7 16 change factor, ver. 1 R_(E″mix) Factor 8.5 2.115 Loss modulus R_(ΔE″) Factor 1.9 1.6 15 change factor, ver. 1R_(E″mix) Factor 6.2 3 13 Transition temperature ° C. 143    140 142calculated by storage modulus, ver. 1 Battery Crosslinking methodContact Contact Contact with with with electrolyte electrolyteelectrolyte solution solution solution until until until initial initialinitial charge- charge- charge- discharge discharge discharge Batterycycle stability 1 % 73   91 85 Battery Internal ° C./sec 5   9 8destruction maximum safety 1 heat release rate Voltagesec >300    >300 >300 reduction (3 V reduction time) Example ExampleExample V-10 V-11 V-12 Separator Resin Polyethylene (A) wt % 96% 58%  0%composition Silane-modified wt %  4% 42% 100%  polyethylene (B) Kneadingtemperature ° C. 220 220 220 Inorganic layer Inorganic Weight 95% 95%95% composition particles ratio Type AlO(OH) AlO(OH) AlO(OH) Resinbinder Tg (° C.) −23 −23 −23 Crosslinking Method — — — methodCrosslinking reaction timing Reagent Temperature ° C. pH Basicproperties Membrane thickness um 11 10.5 9 of resinous microporousPorosity (ii) % 38 42 39 membrane Air permeability sec/100 cm³ 161 180195 Inorganic layer thickness um 4 4 4 Shutdown/rupture Shutdown ° C.153 178 179 resistance temperature (i) Membrane rupture ° C. 160 ≥200≥200 temperature (i) Resin aggregates in microporous membrane /1000 m² 1562 890 Storage modulus R_(ΔE′) Factor 1.1 23 23 change factor, ver. 1R_(E″mix) Factor 1.1 23 23 Loss modulus R_(ΔE″) Factor 1.1 22.5 22.5change factor, ver. 1 R_(E″mix) Factor 1.1 22 22 Transition temperature° C. 151 134 133 calculated by storage modulus, ver. 1 BatteryCrosslinking method Contact Contact Contact with with with electrolyteelectrolyte electrolyte solution solution solution until until untilinitial initial initial charge- charge- charge- discharge dischargedischarge Battery cycle stability 1 % 78 75 70 Battery Internal ° C./sec25 21 19 destruction maximum safety 1 heat release rate Voltage sec 210224 232 reduction (3 V reduction time)

TABLE 16 Comparative Comparative Example V-1 Example V-2 Separator Resincomposition Polyethylene (A) wt % 80% 100%  Silane-modified polyethylene(B) wt % 20%  0% Kneading temperature ° C. 220 220 Inorganic layerInorganic particles Weight ratio — 95% composition Type AlO (OH) Resinbinder Tg (° C.) −23 Crosslinking method Method — — Crosslinkingreaction timing Reagent Temperature ° C. pH Basic properties of resinousMembrane thickness um 11 9.5 microporous membrane Porosity (ii) % 40 38Air permeability sec/100 cm³ 160 172 Inorganic layer thickness um — 4Shutdown/rupture Shutdown temperature (i) ° C. 143 143 resistanceMembrane rupture temperature (i) ° C. ≥200 151 Resin aggregates inmicroporous membrane /1000 m² 2 3 Storage modulus R_(ΔE′) Factor 2.1 —change factor, ver. 1 R_(E′mix) Factor 8.5 — Loss modulus R_(ΔE″) Factor1.9 — change factor, ver. 1 R_(E″mix) Factor 6.2 — Transitiontemperature calculated by storage modulus, ver. 1 ° C. 143 — BatteryCrosslinking method Contact with — electrolyte solution until initialcharge- discharge Battery cycle stability 1 % 55 96 Battery destructionsafety 1 Internal maximum heat release rate ° C./sec 122 235 Voltagereduction (3 V reduction time) sec 6 2<Experiment Group VI>

Porous membranes were formed in the same manner as Examples 1 to 3 andComparative Examples 2 to 3 described in PTL 5 (Japanese UnexaminedPatent Publication No. 2001-176484), and were provided as porousmembranes V-1 to V-5, respectively. Porous membranes V-1 to V-5 wereevaluated for gel fraction (%), heat-resistant temperature (° C.) andneedle puncture strength (gf/25 μm) by the methods described in PTL 5,and the rates of change in the storage modulus and loss modulus RAE′ andRAE″ of the porous membrane V-4 before and after contact with theelectrolyte solution were measured according to <Transition temperaturefor storage modulus and loss modulus (version 1)> in the presentspecification. The results are shown in Table 17.

TABLE 17 Storage modulus change Loss modulus change Heat- Needle factorR_(ΔE′) before factor R_(ΔE″) before Porous Gel resistant puncture andafter contact with and after contact with Membrane PTL 5 fractiontemperature strength electrolyte solution electrolyte solution ExampleExample No. (%) (° C.) (gf/25 μm) (ver. 1) (ver. 1) V-1 Example 1 68 185450 — — V-2 Example 2 69 205 680 — — V-3 Example 3 42 170 460 — — V-4Comparative 36 155 440 Remained at 1 Remained at 1 Example 2 V-5Comparative 80 200 260 — — Example 3

The following is clear from Table 17.

(a) Since the rate of change in the elastic modulus remained at 1 evenwith porous membrane V-4 which had the lowest gel fraction (ComparativeExample 2 of PTL 5, gel fraction: 36%), this demonstrated that porousmembranes V-1 to V-5 had already exhausted crosslinking reaction, andthere was no self-crosslinking (uncrosslinked portions) in the porousmembranes described in PTL 5.(b) Incidentally, Comparative Example 1 of PTL 5 is anon-silane-modified product.(c) The separator of the seventh embodiment of the invention describedabove has value for selective chemical crosslinking of the amorphouszones between the crystal portions. When mixed crystals of anon-silane-modified polyolefin and a silane-modified polyolefin areformed, and the modified units are pushed out into the amorphousportions and become randomly diffused, the adjacent crosslinking unitscome into contact and crosslinking reaction proceeds.

When the multiple crosslinking units become separated from each other,however, they cannot contribute to crosslinking reaction despite thepresence of the crosslinking units. In particular, if (all of) thereaction conditions are satisfied, then crosslinking reaction fromsilanol to siloxane in the porous membrane proceeds immediately,resulting in full crosslinking of the units that are able to participatein crosslinking, thus making it impossible for any further crosslinkingto proceed with the residual units in the battery comprising the porousmembrane.

Therefore, even if residual silanol groups remain in porous membranesV-1 to V-5, crosslinking reaction does not take place inside themembrane-containing battery so long as crosslinking treatment has beencarried out during the step of producing the membranes (that is, theremaining silanol groups cannot contribute to the crosslinkedstructure).

(d) For the separator according to seventh embodiment of the invention,adjustments were made to the molecular weights of the resin startingmaterials, the copolymer concentrations and the mixing ratios, and astretching membrane formation step was also combined, to experimentallydiscover crystal structures with intercrystal distances and crosslinkingunit dispersion distributions that allowed crosslinking reaction of thecrosslinking units to proceed with high probability. It was thuspossible to improve the battery fracture resistance and heat-resistantsafety, and to also inhibit deterioration in battery cycle performancedue to residual hetero functional groups.

The invention claimed is:
 1. A separator for an electricity storagedevice comprising a silane-modified polyolefin, wherein a silanecrosslinking reaction of the silane-modified polyolefin is initiatedwhen the separator for an electricity storage device contacts with anelectrolyte solution.
 2. The separator for an electricity storage deviceaccording to claim 1, wherein the silane-modified polyolefin is not amaster batch resin containing a dehydrating condensation catalyst thatcrosslinks the silane-modified polyolefin.
 3. The separator for anelectricity storage device according to claim 1, wherein the separatorfor an electricity storage device comprises polyethylene in addition tothe silane-modified polyolefin.
 4. The separator for an electricitystorage device according to claim 3, wherein the weight ratio of thesilane-modified polyolefin and the polyethylene (silane-modifiedpolyolefin weight/polyethylene weight) is 0.05/0.95 to 0.40/0.60.
 5. Aseparator for an electricity storage device comprising a silane-modifiedpolyolefin, wherein silane crosslinking reaction of the silane-modifiedpolyolefin takes place when the separator for an electricity storagedevice contacts with an electrolyte solution.
 6. An electricity storagedevice comprising an electrode, the separator for an electricity storagedevice according to claim 1, and a nonaqueous electrolyte solution.
 7. Amethod for producing the separator for an electricity storage deviceaccording to claim 1, wherein the method comprises the following steps:(1) a sheet-forming step in which a mixture of a silane-modifiedpolyolefin, polyethylene and a plasticizer is extruded, cooled tosolidification and cast into a sheet to obtain a sheet; (2) a stretchingstep in which the sheet is stretched at least in a uniaxial direction toobtain a stretched sheet; (3) a porous body-forming step in which theplasticizer is extracted from the stretched sheet in the presence of anextraction solvent, forming pores in the stretched sheet to form aporous body; and (4) a heat treatment step in which the porous body issubjected to heat treatment.
 8. An electricity storage device assemblykit comprising the following two elements: (1) an exterior body housinga laminated stack or wound body of electrodes and the separator for anelectricity storage device according to claim 1; and (2) a containerhousing a nonaqueous electrolyte solution.
 9. The electricity storagedevice assembly kit according to claim 8, wherein the nonaqueouselectrolyte solution includes a fluorine (F)-containing lithium salt.10. The electricity storage device assembly kit according to claim 8,wherein the nonaqueous electrolyte solution includes lithiumhexafluorophosphate (LiPF₆).
 11. The electricity storage device assemblykit according to claim 8, wherein the nonaqueous electrolyte solution isan acid solution and/or a base solution.
 12. A method for producing anelectricity storage device comprising the following steps: a step ofpreparing the electricity storage device assembly kit according to claim8, and a step of contacting the separator for an electricity storagedevice in element (1) of the electricity storage device assembly kitwith the nonaqueous electrolyte solution in element (2), to initiatesilane crosslinking reaction of the silane-modified polyolefin.
 13. Themethod for producing an electricity storage device according to claim12, which further comprises the following steps: a step of connectinglead terminals to the electrodes of element (1), and a step of carryingout at least one cycle of charge-discharge.
 14. An electricity storagedevice comprising an electrode, the separator for an electricity storagedevice according to claim 5, and a nonaqueous electrolyte solution. 15.A method for producing the separator for an electricity storage deviceaccording to claim 5, wherein the method comprises the following steps:(1) a sheet-forming step in which a mixture of a silane-modifiedpolyolefin, polyethylene and a plasticizer is extruded, cooled tosolidification and cast into a sheet to obtain a sheet; (2) a stretchingstep in which the sheet is stretched at least in a uniaxial direction toobtain a stretched sheet; (3) a porous body-forming step in which theplasticizer is extracted from the stretched sheet in the presence of anextraction solvent, forming pores in the stretched sheet to form aporous body; and (4) a heat treatment step in which the porous body issubjected to heat treatment.
 16. An electricity storage device assemblykit comprising the following two elements: (1) an exterior body housinga laminated stack or wound body of electrodes and the separator for anelectricity storage device according to claim 5; and (2) a containerhousing a nonaqueous electrolyte solution.
 17. The electricity storagedevice assembly kit according to claim 16, wherein the nonaqueouselectrolyte solution includes a fluorine (F)-containing lithium salt.18. The electricity storage device assembly kit according to claim 16,wherein the nonaqueous electrolyte solution includes lithiumhexafluorophosphate (LiPF₆).
 19. The electricity storage device assemblykit according to claim 16, wherein the nonaqueous electrolyte solutionis an acid solution and/or a base solution.
 20. A method for producingan electricity storage device comprising the following steps: a step ofpreparing the electricity storage device assembly kit according to claim16, and a step of contacting the separator for an electricity storagedevice in element (1) of the electricity storage device assembly kitwith the nonaqueous electrolyte solution in element (2), to initiatesilane crosslinking reaction of the silane-modified polyolefin.
 21. Themethod for producing an electricity storage device according to claim20, which further comprises the following steps: a step of connectinglead terminals to the electrodes of element (1), and a step of carryingout at least one cycle of charge-discharge.
 22. A method for producing aseparator for an electricity storage device, which comprises thefollowing steps: (1) a sheet-forming step in which a silane-modifiedpolyolefin, polyethylene and a plasticizer are extruded into a sheetusing an extruder, cooled to solidification and shaped into a moldedsheet; (2) a stretching step in which the molded sheet is subjected tobiaxial stretching to a 20-fold to 250-fold area increase to form astretched sheet; (3) a porous body-forming step in which the plasticizeris extracted from the stretched sheet to form a porous body; (4) a heattreatment step in which the porous body is subjected to heat treatmentand subjected to stretching and relaxation in the transverse directionto obtain a heat-treated porous body; (8B) a coating step in which aninorganic porous layer including inorganic particles and a resin binderis formed on at least one surface of the heat-treated porous body toform a silane-crosslinking precursor; and (9) an assembly step in whicha laminated stack or wound body of electrodes and thesilane-crosslinking precursor, and a nonaqueous electrolyte solution,are housed in an exterior body, contacting the silane-crosslinkingprecursor with the nonaqueous electrolyte solution.